Depends where it is really and what time etc for me, I have to be here for Jake when he comes back from school. I don't mind having a look but I'm no expert lol
Things to look out for are the usual really, crash damage, engine noise - if it sounds like a box of nails after running for a little while walk away, likewise if after it is warmed up there is a lot of white smoke from the exhaust, turbo will be on the way out and need replacing.
When test driving go to a straight road (preferably empty) and let the steering wheel loose and see if it pulls to one side this would be tracking etc, then give it a good hard stamp on the brakes to check the brakes....