Check the stickies, someone had a link to a really nice write up.
In case you dont find anything, here is what I'd look for.
Any signs of frame damage. Dents may be one thing, but wrinkles are a sign you should be running away.
Any signs that the forks are bent. If they are, run away.
Fork seals. They should not leak oil.
Chain slop. There should be about an inch of slack. anything over 2 inches should tell you the bike hasnt been cared for too well.
Check the oil drain bolt for small holes drilled in it. Its a sign the bike has been raced.
Any oil leaks, any coolant leaks. If they are getting mixed than its real bad.
If possible see how well the bike starts when its cold. Points to carb problems.
As the bike runs touch a wet rag to each of the headers. They should all cause some steam. If one doesnt, it means the cylinder isnt firing.
Check the electrics, lights, switches, gages...
Thats all I cant think off right now.