Crash damage and blue smoke.
Just for fun, ask the owner how fast hes been in it. Its a good indication whether its been thrashed and trashed, or looked after.
Ill fitting panels, look for pattern panels, eg non VW panels that are not flush with the body and have gaps. Look for odd coloured panels also.
Check the steering wheel. A shiny wheel means its done lots of turns. A non shiny wheel with worn bits indicates lots of motorways (which is better IMHO).
I also found that a worn gearbox has problems with third. Not the gear itself, but actually getting it into gear.
Have fun.
Regards,
Tin