General tips are:
Be a bike geek. Know everything there is to know about the bike you are looking at. Owners clubs, other riders and forums such as this are excellent sources of information.
Make a checklist and go through it. If it fails badly it's probably a dud.
Does the owner have all the paperwork in his name.
Are you at the owners house. Sounds silly but if it's unloaded from a van into a layby ask why?
Ask how long its been owned, check this against the V5, why it is being sold, how much it was ridden etc... get chatting to the owner about his/her riding.
Has it been serviced? Look at the general condition. Is it clean. A good indicator of use are the fork legs. If these have high degrees of salt pitting, the bike has been used in the winter. This may give lie to any claims of dry/summer use only.
Does it start properly, no blue smoke, no coughing, do all the bits (lights, brakes etc) work. And ride it. Check it goes in a straight line, stops properly, pulls well in all gears and has no nasty habits. Away from the owner go over it with a knit comb.
Finally if you have any doubts walk away. There are plenty more motorcycles in the world.
Good luck!
