Usually there is writing along the cable that should say its 6GB/s, if not its 3GB/s.
According to a friend at the local computer store, they are also stiffer compared to 3GB/s cables.
Though in that same store there was a disagreement whether the cable even makes a difference, as apparently all SATA cables are the same.
Just play it safe and use the cables that expressly say 6GB/s on the side (you should get 2 or 3 with the mobo) for your SSD and any SATAIII hard drives you have. Beyond that I wouldn't worry about it.