If it looks right (i.e. all the parts of the bike look as though they match and are of similar age) then check out its history. Does the oil look clean on the dipstick or viewing window. Does it look abused? How many owners? How long has the present owner had it for and why is he selling? That sort of thing.
Also a good tip - when ringing up for the first time always ask about 'the bike you have for sale in xxxx' that way if they respond with "which bike" then they could be a back street dealer. Not always a bad thing but something to be wary of
