Check for signs of rust. Be particular, and inspect to see if the source of rust is from a bend.
Look at it from a bit of a distance to see if it sits right.
The front wheels almost never line up with the rear wheels, so don't worry if it looks like the front end is pushed to the left about 4".
Pick up and drop the bike, listen for a clunking noise. Worn bushings will make it clunk. Compress the suspension and check for a good dampner and good rebound.
Ask the owner to start it. Don't ask to ride it, just ask to see it start, and hear it run.
Listen for a clunking water pump impeller while you're there.