The motherboard or PC manual should tell you. If you don't have it you should be able to download it from the manufacturers web site. Other than that with a free benchmark program like "ATTO" you should be able to tell whether you have it in the right port or not.
For instance if you bought a Sata III SSD capable of 520MB/s and your only getting 275MB/s it's a pritty good guess your in a Sata II port.
For anything over 300MB/s you would have to be in a Sata III port.