The main things to look for are :-
Rust ! look underneath at the sills / in the engine bay and bottoms of the doors particularly.
Unusal noises from the engine, ie knocking, whilsting excessive engine movement when reving car with bonnet up etc.
Clunks / clonks / whines banging noises when taking it for a test drive.
Make sure EVERYTHING electrical works - test and test again.
Dont let the dealer talk to you too much or put the radio on while test driving the car, as they can put you off from listening for noises etc.
One of the most importants things is, is that if the car is around the 72,000 mark, ask if its had its cambelt changed, if not ask if they will do this for you. As if the cambelt snaps on you.. its gonna cost you ALOT to put right.