I once read a really good article on foal conformation, but I can't seem to find it. From what I remember of it (and some of my personal experiences) there are some things that you just can't tell from looking at a foal (better off looking at the parent) and some things that will change in only one direction.
For example: A foal or young horse that is post legged will always be post legged, but one that is sickle hocked or camped out behind can grow to have acceptable hind leg angles. Necks can "become" lower set but not higher (they can tie in lower). Pasterns can get lower but not steeper, so be ware of the foal with low pasterns (exception being a new foal that's "down" on his pasterns/hocks, it's best to judge this on an older foal). Shoulder slope can get steeper but not shallower, so a foal with a steep shoulder will have one as an adult. Major crookedness in the legs (toed-in/out, knocked kneed, cow hocked, etc) can't be corrected, but a small degree of crookedness can be partially corrected with effective trimming if caught early.