as far as what too look for is the usual, look over the frame ask how many hrs are on motor, has it been rebuilt, maintance, etc? alot of people will tell you to test the compression but this can be a pain in the butt and i dont think many people actually take out a comp. tester and check it?
start the bike see how she runs, maybe take a test drive.
good luck with it does it have any mods?