Originally posted by: Modular
Do you mean while playing? If so then FRAPS is the way to go. Use the free version.
Otherwise, use the built in benchmarks.
Originally posted by: nitromullet
If you want to just view the FPS while playing, just do the following:
open the console by pressing the ~ key
type: "r_displayinfo = 1" without quotes
"r_displayinfo = 0" turns it back off.
This also tells you what path (DX9/DX10), game version (32/64-bit) you're running, and is a good way to check if SLI/CF is working properly which will be indicated by "MGPU" in the info.