Ask for proof of purchase, turn it upside down and look for Bike register police marking by the bottom bracket, take a mate, get a vibe about the seller
Look for damage on the frame, look at the state of the chain, mech, ask when it was last service, look at the tyres. Let the wheels spin freely and watch for wobble. Test ride it
What year model is it and how much is it up for? Post a link, let someone look at the photos (and the description of the bike or lack of, can be very telling of a dodgy one)
I'd be worried because C'dale Bad Boys are painted black
