If you know for sure what type of bike you are going for, I would suggest spending the £15 on the Haynes Manual in advance and haveing a look through it.
I have always done this (for cars too) and it has always payed off for me. I also take a friend with me who know's his stuff and it's good to have a second opinion as it is easy to get carried away with yourself.
Tools wise - Torch, Small pincer screwdriver (for poking stuff), rubber gloves (2 or 3 pairs, allowes you to dig right in without getting the car dirty on the way home).
Take a test ride and push the bike hard through the gears. Listen for noises. Emergency stop, then andother one from a higher speed. Check the bar ends and pegs for signs of a spill. Chain, sprockets, pads, oil level & colour, suspention, sevicing and general condition. If it don't feel right, walk away. Do not leave yourself stranded miles from home with buying a duffer as your only form of transpost. If you like it and she's good, haggle...And do not pay a penny more than you had decided before you leave the house