Perhaps even worse than buying the wrong bike is buying the right bike and having the top-end frag on the ride home.
No, you can’t check everything. And even if you could, pristine, well-maintained and carefully ridden bikes have been known to reach an early end.
There are some things you want to look at closely, though. Depending on your time and mechanical skills, these can be negotiating points or reasons to pass.
Know what you want. Web-based discussion forums for your model or type of riding will be the best source of information. These people are real riders who have spent real money maintaining their bikes. Expect to find detailed knowledge (and strong opinions) about model-specific concerns.
Check the maintenance records. Sure, these can be fudged, but when you buy a used motorcycle, some level of trust must be assumed. If physical records don’t exist, ask for a verbal account of maintenance history.
Examine tire wear. Look for left side wear on both tires (right-side driving means more distance is covered turning left) and center wear in the rear. You want at least 1/16 of depth in the tread. Check the tire pressure while you’re down there. Improperly inflated tires may hint at lazy maintenance habits elsewhere.
Test for excessive bearing play. Ask the seller to put the bike up on the centerstand. Gently tug on the fork tubes front to back, feeling for play in the steering head. Also feel for movement in the front and rear wheels and side-to-side action in the swingarm. Basically, any play is bad and a safety hazard.
Check the steering head lock. Does it work? Use the key and find out for yourself, but just as important, look for any signs of damage or excessive wear. Has it been replaced or repaired? If so, the bike may have been stolen.
Examine the air filter. It should be clean and properly installed. Consider a zero-tolerance policy. Dirt in the engine is a very bad thing. Also, is the airbox intact? Extra holes (punchouts) could be fine (as long as they are outside the filter), but ask the seller why they were performed.
Check all fluid levels. Discolored brake fluid, low coolant levels and dirty (or gray) oil are all bad signs. Don’t forget that some bikes have separate crankcase and transmission oil—and two dipsticks.
Perform a visual once-over. Look for any obvious mechanical issues—loose or missing fasteners, fluid leaks, pitting in fork tubes, rust in the fuel tank, cable continuity, chain play, sprocket condition, cracked wiring, etc.
Perform another visual once-over. This time, focus on cosmetics with an eye toward identifying crash signs. Look for inconsistencies in paint shade or texture. Cracks in plastic or fiberglass may be invisible from the outside but obvious from the inside. Use a flashlight and check all the cracks and crevices. Examine footpegs and sidecases for scrapes. Do the control levers appear extra-shiny? Ask why and when they were replaced.
Operate all controls. While the bike is still on the centerstand, give it a “dry run.” Test the brakes, operate the clutch and take note of sufficient play, shift gears, flip the turn signals, beep the horn.
Time for a test ride. Don’t get offended if you’re shot down. Many bikes have gone out for test rides and have not come back, so sellers have good reason to be wary. A reasonable alternative is asking for a “first-ride-cash-back” guarantee—you hand over the cash, the seller hands over the keys and you have an hour or so to return the bike, damage free, if you’re not satisfied.