Unless they are high grade, they are only worth a few cents more than face. As for "what to look for", that is a very broad answer.
A high grade red 1935-S could bring a nice premium. Circulated examples for both dates and MM are worth a few cents over face.
As the FrogMan told you, they need to be higher grade. Go to NGC or PCGS and check their price guides for the decade you are interested in, that will give you a good idea. Or spend some bux and get a Yeoman Red Book. Spark