This is actually fairly easy for emergent readers because there are plenty of "leveled" books for them such as Frog and Toad, and Owl at Home. Early readers are labeled by the publishing company as being part of a series of steps clearly explained somewhere on the book. The "I Can Read" system, for example, has books labeled as "My First" which are suggested as books good for "shared Reading" and then has four more reading levels with more and more advanced vocabulary, word complexity, and sentence structure. Often, the highest "level" in these types of systems are early chapter books. They have a contents and are divided into chapter sections, but are still fairly short stories with illustrations at least every few pages. Each major publishing house that produces these types of books offers its own leveled system, but they are all clearly labeled and explained right on the books and they are also all fairly similar. After your child moves into "chapter books" that are no longer considered "beginner chapter books" however, it can get a bit more complex to find reading for a child that is at just the right level.
Readers Past "Beginner Books" but still needing to read Children's or Young Adult Books
While there is not a good system for rating the "content" of a book for children, there is a "rating system" for reading level or difficulty of books written for children and young adults. This system at least helps to determine what a child is likely to be capable of understanding even if it can't guide a parent in knowing fully whether any particular book is just right for that parent's own child in terms of potential interest as well as maturity of the content. Many books are registered with an "RL" on them that can be found near the bar-code or on the copyright page. If not there, you can often look it up online through the publisher.
In the case of Hitchhiker's Guide to the Galaxy, the RL is 8.3 (Scholastic). Again, I remind all that an RL is an indication of what average grade level (in the US) will understand at least 80% of what is being read - and in no way reflects the content covered in any given book. For a book with an RL of 8.3, an eighth grader about three tenths the way through his/her eighth grade year would be expected to understand at least about 80% of what he/she reads. It also may not be helpful as grade level is different in other countries, but there is a more internationally minded answer to the question as well.
Lexile measures are another way to gauge the difficulty level of reading in a book and measure approximately the same thing - they just go about it a different way. HHGTTG is measured at 1000L. For the sake of comparison, The Hobbit is also measured at 1000L.
There are other systems for choosing reading levels as well, but I am less familiar with most of them as I find them as having ranges that are a little wider than I'd like and I have just found Lexile and its website so easy to use I use it almost exclusively.
In terms of determining specifically whether any individual child is ready for any particular book or not
Readiness will depend much on how an individual's reading level compares with that of the book. If you look up the Lexiles for other books the child/adolescent has read and understood successfully, you can determine if his/her score would most likely be above or below the Lexile measure for any given book.
In order to use these numbers though, it is important to understand one key caveat - content matters too and the numbers don't figure it in. So first, I'll explain How these numbers are determined
In the Flesh Kincaid Reading Level system (RL's) it is a simple calculation considering length of words and number of words in a sentence and then compared against an average based on a set standard using abilities of children at each grade level. A book with short words and short sentences will have a low RL but a book with longer words and more words per sentence will have a higher RL.
The Lexile scores are slightly different because it also considers the repetition of words as making a text easier to read. For this reason, the reappearance of the same words is also calculated into the score. So books with shorter words, used more often and in shorter sentences will have a lower lexile while books with long sentences, long words and few words repeated will have a much higher lexile measure.
Considering all of this, for many kids an RL will often be listed as higher than what many kids can handle at that grade level and in the case of Lexile, you should consider your child as existing with in a score "range", so unless your child is considered a "remedial reader" you are likely to find the numbers a little on the high side because they are averages. Giving a fourth grader a book with an RL of 5 is probably not going to mean the book is out of his or her comprehension reach.
At the same time, if the book is about a subject that is brand-new to your child, it may be harder to read than the numbers indicate so for a first foray into a new genre (or topic if it is non-fiction), I suggest choosing books with a score lower than your child's reading level as an introduction. Also, books that are highly sarcastic in nature (Such as the Hitchhiker's Guide to the Galaxy) might be a little trickier in terms of the understanding of the jokes themselves.
For reasons of content it is also important to know the child who is doing the reading and that child's available background knowledge when offering a book for reading that is of a higher number than the child's reading level. Books with higher scores than that of the child, should be pushing reading ability but not content knowledge. I frequently give my child books above her reading level, but only ones in genres she is already familiar and about subject matter with which she is also already familiar. Otherwise the exercise of reading the book becomes a matter of frustration for her rather than a fun read. Scholastic's "interest levels" can be helpful in determining this aspect of books, but not all publishers offer such a number and it is not as scientifically measured (or measurable) so I feel reading the summary and giving a book that you have questions about a good skim-through is the best way to help use the Reading Level score you have most effectively when in question about a book.
Advanced Readers Delving into Adult Literature
You can still find an RL or Lexile score for much of the published adult literature if it might be used in classrooms or is considered a "classic." In these cases you can use the same methods in finding appropriate "RL's" as listed in the section above.
For example, Jane Austin's Sense and Sensibility is listed as having an RL of 10 and an 1180L or The Grapes of Wrath is listed as RL 4.9 and 680L (However, the content is more challenging so its "interest level as determined by Scholastic is 9-12). Hamlet, Macbeth and King Lear by William Shakespeare are listed in a book together with an RL of 11 - however, a Lexile score is not currently available.
In those cases where an RL or Lexile measure (or your preferred system for determining Reading Level) is not easily available, you can take a look at reviews of the book and check out the publisher's description of the book. However, since most persons at this reading level are nearly adults, I might also consider just trusting that the teen can come to you and ask about confusing aspects of the book (if you have a close relationship) or figure if it matters to him/her your teen will figure it out.
If you are choosing books for educational purposes, you can also search the book title and "book club" and often come up with a great summary, sometimes a vocabulary list and often great supplementary information and discussion questions before you go ahead and do your pre-reading.
As a suggestion for when you still aren't sure:
Whenever I am unsure if my child is ready or not for a book (but I think she is close), and she doesn't want to wait, or the content of the book is relevant to something she is studying now (so waiting doesn't make as much sense), we read it together. It is a fun way to bond, we can discuss what we are reading if she gets stuck or doesn't understand something. Plus, generally, they understand what they hear better than what they read, so you can read the portions that contain passages that are more difficult, and it allows me to remember all the details of the book and enjoy it a second (or third, or fourth . . .) time around. This method is one I use both for when I am unsure of the reading level/complexity of a book for my daughter as well as for when I am unsure about the content.
In order to round out this answer, I thought I'd offer a few other examples for the Sake of Comparison:
Anne of Green Gables has an RL of 5.9, but in terms of content might be most interesting for someone in grades 6-8, or ages 11 to 13/14 or so (it is a coming of age story, so adolescence is generally a good time for reading it). It also has a Lexile measure of 990.
Harry Potter and the Sorcerer's Stone is considered to have an interest level for most kids around fourth grade and up through middle school (about 9years old to 13 or 14). The RL is 6 and the Lexile is 880 (in fact a couple of the books share this number, 880). Deathly Hallows tops the Lexile score for this series at 980 and an RL of 7.4.
Number the Stars by Lois Lowry has an interest level listed by the publisher as grades 3 to 5 (age 8-10) and an RL of 5.1 with a Lexile measure of 670L.
The Lion, The Witch and the Wardrobe is also listed with an interest level of 3-5 or ages 8 - 10, but has an RL of 6.1 and a Lexile of 940L. Prince Caspian comes in with RL 5.6 and 870L while The Last Battle is RL 5.8 and 890L.
A Wrinkle in Time has an interest level at 6-8th grade, an RL of 4.7 and 740L, A Wind in the Door comes in at Rl 5.3 and 790L. A Swiftly Tilting Planet is RL 6.4 and 850L.