Ask lots of questions about ownership. Do they have an original receipt, why are they selling, do they sound like they know the bike, or are their answers generally vague?
If it's on an online auction site is there lots of detailed pics, or just a random, poorly taken pic outside a pub/shop/park bench?. Is the description detailed, or does it say things like Shimano gears rather than Shimano Tiagra 9 speed gearing for example?
If you ask for the frame number you can also check it against databases such as
THIS one.
If the seller won't give you the frame number, or says there isn't one, or you just feel they are being vague then simply walk away would be my advice, there are loads of genuine sellers out there so why take a risk?