Interesting how the year and model weren't mentioned, just the brand and type (CX). If you check it out, turn that thing upside down and check for the serial number that should be located on the bottom bracket housing. If the seller asks why you are flipping the bike over, just tell 'em you're just checking for any visible cracks in the frame down there. If that S/N is missing or looks like someone tried to scratch it out, odds are that bike is not for sale by the proper owner.