I'm not aware of any In-game command to show Frames per second in EQ2, but you could use a program called FRAPS that works with just about any game and will show you your FPS. You can get it at
www.fraps.com.
However, I am not sure if use of this program with EQ2 breaks the EULA, since Sony has never had much love for third party tools. I don't see why it would, since it doesn't enhance the game anyway nor give you any unfair advantage.