Movies have ratings, which vary a little around the world, but the ratings just don't convey a full understanding of some scenes which may be scary for young children. For example:
Monsters Inc. is about humorous, fluffy monsters that live in another world, but their day is to scare the living daylights out of small children to 'harness the screams'. Hence there's many 'boo' scenes that are pretty frightening if you're 2 years old and you don't know they're coming.
Sleeping Beauty is a beautiful/classic Disney movie with lovely visuals and nice songs, but has the wickedest, meanest most scary evil witch of all the ones I can remember. Go watch it again if you don't remember — Malificent is her name. Really mean, really scary — the stuff of nightmares.
Both of these movies are rated 'U — Universal'.
So, my question is, how can you determine that the movie is suitable for a 2, 3 or 4-year-old child?