leaves, in general

for people who are newly growing...

your leaves are not going to look perfect forever. If they droop, they probably need water, or it's hot and they're busy expiring [as in breathing not as in dying ]
your leaves will become more ragged/blotchy/less perfect looking as they get older, new leaves will pretty much look good unless they are eaten by snails, in which case there'll be bits missing out of them or they will have disappeared entirely.
if they're yellow, they are either drying out or will be lacking in nutrients [if they're yellow all over the plant] or just old and they've had enough [if they're at the bottom of the plant and the plants have been alive for more than 6 weeks or so]
they will look lumpy or crumpled, the ends will flop over, the undersides will be purple [from cold for tomatoes]
if you have light yellow crumply bits, the plant has probably been scorched.
if they are ok but have black sooty marks, it's blight, but that won't happen until at least june.
um...can't think of anything else offhand but I'll probably be back or someone else will