Originally Posted by
SkyDog75
If by 'legit', you mean authentic...
The serial number should be identifiable.
Compare the bike to Cannondale catalogs for that model year. Does the frame construction look right? Do the color and decals match? Are the components the same -- crankset, fork, derailleurs, shifters, brakes, etc.?
Components could have been changed over by a shop, the owner, or even a previous owner, so this is not always a good way to tell. Some shops even change the builds that come out of the box.
Best way to tell is to meet the seller at a shop, preferably one that sells Cannondale if possible. Have a mechanic take a quick look at it, which most shops will do.