Hello.
You can use to methods to monitor fps in game:
1.- If you have activated Visual Mod via JSGME, you can display the main menu. You will need to assign a key under Controls to open the menu (it is under the name 'Game Menu' in the Misc section, between 'Toggle Mirrors/Show Mode' and 'LeavePlane'. If you didn't activated Visual Mod, this entry won't be visible). In game, you just enter the assigned key and a menu will pop up. Choose Options > FPS.
2.- If you don't have Visual Mod activated, you can open the console (SHIFT + TAB), type 'fps START SHOW' (without quotation marks) and leave the console (SHIFT + TAB again). FPS will be displayed in the same format as in the previous option. To hide fps info, just go back to the console and type 'fps STOP SHOW'.
Cheers