May be the owner of my first FZS600 Fazer

[nick crisp]

Ask him not to warm it up before you get there! See how it starts from cold - battery good and doesn't have carb issues etc? Put your hand on the engine when you get there, to check he hasn't. If he's warmed it up anyway, it may give you a clue that he doesn't want you to know something about it, then be suspicious about everything else he tells you! And of course, any service history he can show you may help to see if it has had any problems, indicate whether or not it's been properly cared for (although it's not definitive). Compare indicated mileage with wear and tear that you can see - more difficult to decide about on older bikes with higher mileages tho.
Good luck, and welcome to the forum