I guess you haven't anyone to take along with you....
This is what I would look for
Is it well cared for? I mean, if the tyres are worn or the horn doesn't work, or there's a rip in the seat, then it's been bit short on love, and that can mean it's packaged up with more serious problems.
What's it like when it starts. It should start on the button and the exhaust shouldn't smoke. You need to check that the engine isn't warm before you start it; sellers can sometimes get the bike started before a buyer visiting, which helps to hide problems.
Once it's been running for a few minutes, open up the oil filler cap, and look at the inside of the cap and around the rim of the engine. What colour is the oil? Ideally it's a pleasant luminous caramel colour, but it could be blacker than that, depending on when the oil was last changed (ask about its service history, but be prepared to take the information with a pinch of salt). If the oil's at all creamy or buttery, walk away.
Is this is kind of information you were after? If so, I'm sure others will add more useful stuff than I can give you!