I did a quick search on Google and found this:
Check Starcraft 2 FPS on Mac
To check the frame rate on a Mac, while you are in a game hit: Control+Option+F
Show Starcraft 2 FPS on Windows
Checking FPS in Windows is basically the same key command, in SC2 hit: Control+Alt+F
On either operating system, once you hit the command to display frame rate, the FPS counter will show in the upper left corner