From the web (and this sure ain't all of it):
Used Dirt Bike Checklist
Check the obvious things like broken plastics, a torn seat cover, bent handle bars, worn tyres, broken clutch and brake levers, and a worn chain and sprockets.
Make sure the chassis number has not been scratched off... this indicates a stolen bike.
Check the (wheel) rims for cracks and bends.
Check the frame/chassis - look for cracks, fractures and old repair jobs.
How worn are the (brake) discs? If there are noticeable grooves where the pads touch - the bike has been well used.
Is the kickstart sloppy? If so, this is another sign of a well used bike. Look for fractures also - new ones are not cheap.
Check the radiator for bent cooling fins and cracks. Make sure it's topped up and not leaking.
Make sure the front brakeline is not worn from the guide on the front fork. If it is, this needs to be replaced.
Put the bike on a stand with the rear wheel off the ground. Try and move the rear of the swingarm from side to side to check the swingarm bearings. There should be very minimal movement here - no more than 4-5mm. If there is, the bearings will need replacing.
Gripping the tyre like a steering wheel - wriggle the front and rear wheels to check the wheel bearings. You don't really want any movement here.
Hold onto the lower end of the front forks with the wheel off the ground and try move them back and forward. If you feel a bit of movement in the steering head, the steering head bearings may be on their way out too... $$$
Check the fork seals are not leaking. You will see oil traces down the front forks if they are.
Everything looking good so far? It's time to take it for a spin!
When starting the bike, check the compression in the kick start. If the piston is worn out, the down stroke will be soft and effortless. Unless you have ridden a number of bikes this will be difficult to gauge though. Especially on different sized engines.
Next, listen to the engine. If it makes loud, unusual tapping noises, then it could mean the motor's well worn and requires a rebuild. Again, this can only be picked up by someone with experience.
Now for the fun part! After a decent warm up... give it s#@! You need to be able to open it right up to really see how it runs. So make sure you have a decent area to test her out in.
Make sure you feel comfortable with its power delivery and 'rideability'. After all... a tweaked-out, peaky, two-stroke rocket isn't necessarily going to make you ride better... and you certainly don't want a smoke-billowing slug.
_________________________________________
Don't take all of the above as gospel. It gives you an idea of what to look for. A swingarm that has 4-5mm of slop in it has been shot-to-hell for quite awhile!
Check the final drive, too. A chain and sprocket set is spendy...pushing $180-200. Pull the chain away from the rear sprocket AT the back of the sprocket. If you can pull the chain 1/2 a link away from the sprocket...the final drive is shot. The KDX200 came with a non-ringed chain. It's a piece of crap that might last for a few rides. If the chain is ringed you know it's been replaced at least once.
Tires maybe $60/ea..and that doesn't count mounting 'em if you don't do it youself...and the new tubes you will want to put in.
The honda is a maintenance whore....
