I've heard a figure of someone putting 90k miles on their TW if that tells you anything. There are a few decent "what to look for when buying a used motorcycle" write ups on the web I've seen. The most important thing you can check is the engine case! Check it well for any existing cracks or non factory welds that may have been made on it, if there's anything of that nature then WALK. Secondly I won't buy a bike without some kind of test drive, if they're not comfortable with it then neither am I in buying it. Check the brake life indicator on the rear and the chain adjuster position, these are items you can tell right away if you'll be having to replace. Check the steering column for play or wobble or anything like that that doesn't feel solid, also test the suspension to see if it's responsive. Look at the forks for any bends or dents as well as the rims, check all the lights, turn signals, speedo, odo, trip and the horn. Check all the rubber hoses, plastics and their connection points, the tires and the fork boots for any dry rot or old age cracking and such. My sons '01 xr80 needed all the rubber and plastics replaced cause of the age of them. Live and learn. As far as the valves go I don't know the symptoms of being out of adjustment but the adjustment job is a simple 15-20 minute deal. There's a great detailed "how to" thread of it here. Read up on some of those articles on the web, they seemed helpful and thorough to me. Other than having a mechanic check the bike out there's only so much you can tell from the superficial examination I suppose. These TW's are a breeze to work on though if need be and the price is right.