This going to be more 'generic' when looking at used bikes/cars/etc.
Fuel/carb/injector:
a.When startup occurs, the fire off should be instant. That says; no injector clogged/no fuel circuits clogged in the carb.
b. When the bike idles smoothly this says; air screw has not been tampered with/low jet not clogged/main jet not clogged. Hard starting says one of the carb circuits are clogged.
c. So, poor starting/stalling/won't rev out; points to fuel system problems to watch out for.
Cold Engine Smoke:
a. Always walk up to a cold engine when buying used. Say you'll come tomorrow and look at it again, so don't have the seller sell you a warm engine.
b. Always let the seller start a [cold] engine. You then stand behind the bike/car and look for smoke out of the muffler. This tells you the engine needs a major overhaul and machining.
c. Always remember; smoke upon startup and clears = Worn Valve Guides. Constant smoke and does not go away = Rings.
Compression Test:
a. It's all about your hard earned money used is about to piss away, so you more bring tools to the close the deal or renegotiate.
b. It's all about the health of the engine, so you bring a spark plug tool and a compression tester. You then ask the seller to close the deal or walk on the final inspection. If there is a 10-15% change between cylinders, here is your poor idle with clean carb circuits. This may be why the seller is selling the bike, who knows? At least you made the offer to show the owner what condition the engine is in.
c. It's all about fuel-idle clue, spark-brain box works, and compression-pressure power. This takes care of the engine inspection.
Chassis:
a. You want to look at the fork stops and note their condition. If one tab is bent or has an indent, it's been dropped/crashed.
b. You ride the bike and the bars go one way and the front wheel goes the other [straight meaning]; the upper and lower crowns are bent.
c. You need to stand in front of the bike [being upright] and look at the front end to see if the frame's neck is bent.
d. You turn around and walk away from this bike, that or buy it as a donor bike on the cheap.
e. You take a wire coat hanger and bend a straight wire so the ends touch each other. Now, bend it back straight. See the "S" bend as you tried to straighten out the neck, the lower tree and that stem, fork tube, my dick, you name it.
f. You buy new parts instead; instead of salvaging the part and lose that money towards new = Cha-Ching! Buy hair beware!
Drivetrain:
a. Say you're looking at a chain drive. The visual is, how well lubed is it?
b. Say you grab a link at 3 o'clock on the sprocket and pull it back away from the tooth. Call that X movement to see a worn chain. Y movement is to push the same link up and down so the whole chain rides up and down in the U's of the sprocket; and that tells you how worn the teeth are.
c. Say you begin to add this up and it becomes a whole unit to change, as you do not match new over old kind of squidshitactic.
d. Say the tires are a given, be it old or worn, so do you see the price of the bike vs. an arm and a leg for a worn piece of shit?
Hangers:
a. That says things like fender tip damage, winker lens/housings showing a crash/drop.
b. That also says the levers and scratches on those; due to crash damage.
c. That about covers the quick and dirty of buying junk or buying a bike.