Check for oil leaks under the bike
feel the exhaust (in front of motor) BE CAREFULL MAY BE HOT!! before you start the bike if they are hot then he has prerun the bike to hide any cold start issues. And if he says that someone was there just a little bit ago test driving it (BULLS#&T) wait till the bike is cold then start it.
Check the throttle response it should be crisp and fast make sure nothing is broken around the throttle area/hand grip.
feel the brakerotors if they feel like an old Elvis record that's not good you can also see how the brake pads are.
check all the lights, horn, starter etc. and make sure all electrical stuff works.
open the gas tank and look inside make sure there is no rust on the inside of the tank. Smell around the carbs shouldn't have a strong smell of gas.
Check for fuel leaks and make sure all the Vacum lines are in good condition.
Listen when the bike is at an idle. (the bike should be idling at about 1000rpm)
any higher and he may be hiding primary chain noise or cam chain noise.
motor and bike should sound smooth other then the nice ticky ticky from the valves. That's a good thing. Bike should accelerate smoothly and hold speed and not lunge, Hunt, surge, sputter, backfire or spit!
Run the bike through all the gears (it should shift smooth) make sure it doesn't jump out of gear.
when you grab the front break it should stop smooth and not have any vibration.
FIRST AND FORMOST!!
write down a check list
and go through it!
If this person tries to rush you or says "I gotta leave in 15 minutes" WALK AWAY!! he's trying to get you to buy a piece of junk.
those are just some things I would check but didn't on my first bike.
