How do you find your fps?

I'm pretty new to all of this video game stuff, and all I hear is everybody talking about their fps in doom 3, just wondering how to check it. I have a 128mb NVidia GeForce FX 5200 card.

http://www.doomworld.com/vb/showthread.php?s=&threadid=26574

Please search the forums for your answer first before creating a new thread.

Thanks.