I think all babies look alike, even when they are not identical. I need to come up with a strategy over the next few months.
My wife and I faced this very issue with our twin boys, now four years old. When we first brought them home from the hospital, we kept the little wristbands on them for the first few weeks, until they started to outgrow them. Then, we painted their big toe a different color so that we had a foolproof way of keeping track -- the nail polish wouldn't come off easily, and even when it did start to come off, it would generally leave traces of the color for a while. After a few months we managed to identify tiny differences (small moles, different patterns to the way their hair swirled, etc.) that we could use, and once we were confident that the differences were there to stay, we stopped painting their nails.
I studied philosophy in college and my wife is a linguist, so besides being two people from professions that should never be permitted to raise identical twins (physicists being another group that should also be so banned, since their textbooks are similarly filled with "thought experiments" that would more aptly be characterized as "twin torture") we were also quite obsessed with the existential implications of how names work, and what happens if they get mixed up.
If you do go with the nail polish approach I'd definitely make sure to get a brand that's non-toxic and to not over-do it because little growing nails do need to breathe -- you might want to switch up which nail you paint each time. Also -- and this is very, very important -- you need to reach an agreement with your partner that neither of you will ever, ever, EVER "play a joke" on the other and switch them on purpose. :)
Not a parent of an identical twin here- but one myself. My brother and I are so identical that even our parents and five (yes, five) older brothers can't tell us apart quite frequently. Of course, we purposely wear similar clothing and are very similar in personality, as well.
When we were born, our parents were prepared for twins and actually just grabbed a nontoxic marker and drew a little star right on my brother's (the older one) hand, and kept it that way for several months.
An unrelated fact is that for some reason, we have the same first name. And actually go by that name half of the time, though we do use our middle names as well.
Even identical twins aren't exact copies of each other. There ARE differences!
Look for these differences, however small, and remember that Bob is the one with the narrower toenail, and Ben was born with longer hair. Alice has a dimple on the left butt cheek, Denise has one on the right shoulder.
If all else fails, you can still mark the twins as @Bill explains.
To add to the above comments use clothing colors and patterns. The following list developed over time, often based on what colors the our boys were drawn to at a young age. Since about 3 months old Twin B always reaches towards the yellow toys and clothing.
In our house it is an unwritten rule that:
Twin A always wears blues, greens, browns, solid colors, and/or anything with puppies, trucks, or superman on it
Twin B always wears yellows, reds, oranges, stripes, and/or anything with monkeys, sports equipment, or batman on it.
When the twins are first born and their footprints are made, these are one of your best guarantees of uniqueness. These will also be a treasure for your children, so keep these and perhaps make new ones over time. It takes a moment and it's much easier than stressing over how they are dressed or otherwise decorated.
I have triplets, two of which are identical. It is incredibly unlikely that it will be impossible to tell them apart. If they share a placenta it is likely that they will be born with different weights and will always be slightly different.
After mine were born the smaller one was intubated once and had an umbilical vein catheter placed - so the cry (voice) is different and the navel is different. It didn't take long to find a lot of differences, such as freckles. Babies are also usually born with skin blotches created in the womb that fade later. One of our identicals had a huge dot on his forehead.
When they are born just inspect their skin very carefully and you will probably find distinguishing marks. Otherwise go with the markings.
When they are old enough give them different haircuts. We had one long, one short for a very long time until they wanted the same haircut shortly after turning five. They're still five and we're tired of that - they are getting disssimilar haircuts as soon as possible.
One thing that always concerned me was nail polish. Some nail polish has chemicals that are teratogenic - as in causing birth defects. I wouldn't use anything like that on a newborn.
If you do use markings, use something that will survive a bath.