You're going to have to rely on mostly visual inspection. Bounce the fork as much as you can to feel/listen for binding. Look for leaks at the seals. Look for cracks or other signs of stress on the frame.
Engine/transmission leaks can occur at quite a few points, but they should be obvious unless the bike's just been cleaned. I wouldn't worry too much about that. Most can be fixed fairly easily.
Chain slack isn't really an issue unless it's way too tight, in which case engine bearings may have been damaged. Rather, check for general condition and tight links indicating the need for replacement. Notice where in the adjustment range the rear axle sits. If it's near the back, the chain may be "stretched" and due for replacement. Look closely at the sprocket. A new chain and sprockets might go over $200, so use that to bargain if appropriate.
Look at the tires and other soft parts like hoses, wiring, and spark plug boots for cracking or wear. A 14 year old bike could be due for some replacements.
Listen to a cold startup, then take it for a ride. Try to get a feel for the owner's (and previous owners') maintenance habits.
Check
this thread in the Nutshell for more.