Is there a way to tell if I can use a SATA III SSD in my early 2011 MBP 15"?
I have the low-end MBP 15'' early 2011.

It has a SATA3 interface. As I have a SSD build-in, the negotiated link speed is 6Gb/s as well. Benchmarks achieve speeds of up to 510 MB/s, which is clearly SATA3.

Also, make sure that you have the latest firmware installed. The early 2011 MBP received firmware updates which enabled full link speed (as of revision 2.2).