Have a plan for what you want and a realistic price.
Make sure the bike fits your criteria
Check compression( hand on tailpipes is useless, thumb on spark port is useless as well)
Grab the front end and yank it every direction from several points of the left and right suspension areas. Listen for squeaks and rattles, feel for play or crunchy irregular movement.
Grab the rear and tweak on the axle, the carrier, the wheels/hubs, swing arm.
Look for hillbilly modifications
Hardware is in good condition
Things are tightened down properly
Cracks in frame
Wear items haven't been neglected
Is the bike clean
What is the condition of the fluids
Can the owner speak with common sense, describe the bike and it's history in detail.