Here are some tips...
If you're primarily worried about the motor, see if you can take it to a shop to have a leakdown test performed ($50 - $100). This will measure the general health of each combustion chamber by pressurizing the cylinder, and measuring how fast it leaks. 10% or better (less) is ideal, up to 15% on high mileage would be acceptable, anything more means it's getting weak. All cylinders should leak down about the same. If there is excessive leak down, you can usually see right then whether it's valves or rings or both - that means $$$$.
But some other things to look for include:
- look over the general condition - is it clean, does the chain have lube on it, is it adjusted correctly. Is there oil weeping past the fork seals.
- have someone hold the bike, and hold each wheel and shake side to side, there should be no play in the wheel bearings. The wheels should roll with no rough spots.
- check brake caliper mount bolts, drain plugs, and axle pinch bolts for saftey wire drill holes. If it's been safety wired, it's been on the track.
- if the tire sidewalls are cracked, they're toast. If the edges of the tread are blue, that means they've been heated up pretty good at one point, and may not have the adhesion they should. Check the wear patterns for cupping in the front (under-inflation). Make sure there are caps on the valve stems - centrifugal force can cause the valve cores to unseat at speed and the tire to lose air.
- check the chain for kinks, loose rollers - it shouldn't squeak or rattle.
- pull on the chain on the back of the rear sprocket - if you can see half a tooth, it's toast.
- check sprockets for hooked teeth.
- is the coolant the right color (usually green), and does it have any oil or rust in it.
- is the brake fluid in the reservoirs clear and light colored. They shouldn't be milky (water) or dark (old).
- does the suspension operate smoothly, does it sqeak or click (it shouldn't). Does the rear shock have any rebound damping left.
- when pushing on the front with the brake on, the steering head should feel rock solid.
- how much brake pad is left (front and rear).
- any oil leaks on the engine.
- how does the engine sound (when cold and warm). Some engines have some funny sounds to them, but there should be no rattles (cam chain), or knocks (main or con-rod bearings). Light taps may mean a need to a valve adjustment.
- does it rev cleanly, or does it miss (crap in the carbs, bad jetting).
- do they have receipts for work.
- does it have the stock exhaust or an aftermarket. Aftermarkets are great for power (if installed correctly and jetted), but create more noise, and usually corrode faster.
And finally - when you get it, yes change the oil with a brand made for bikes (Honda, Yamalube...), and I recommend the manufacturers oil filter over an aftermarket. Way too long a post - have fun and good luck!!!!!