There are 4 things to look for when buying used tires:
-They hold air. This is the hardest to check, because a slow leak may take a long time to show. Its a gamble, but I have bought tons of used tires and never had one leak.
-They have tread. Measure it with a depth guage, pair of calipers, a penny, whatever. Piece of cake.
-They aren't too old. The date of manufacture is molded into the tire. Piece of cake.
-No obvious gouges in the sidewalls. Just look them over.
Sure, you may get a bad one once in a while, but its so uncommon, and used tires are SO much cheaper than new ones, its worth a shot. If you get them from a used-tire shop, they will usually have a guarantee against a slipped belt, leak, excessive runout, etc.