Hit the ~ button to bring up the console screen. Then type "timedemo 1" and hit enter (without the quotes). Then type "demo demoxxx" and hit enter (once again without the quotes) where the x's are replaced by a demo number (e.g. demo001 or demo127). For some reason I can't get demo001 or demo002 to work with the version I'm using which is 1.27 g, but demo127 works fine. The game will play as fast as it can. When it finishes, hit the ~ button again to see your frame rates.