As far as I know, anything resembling a console has been removed from this game, so it's not an easy matter. There is a long, complicated process involved in order to get cheats to work that MAY also work to be able to read FPS, but any saved games affected by your measurement will then be branded as 'cheat-enabled'. Since Deus Ex is an Unreal Tournament-based game, then framerates will be almost identical in the two games (on indoor levels, anyway). If you have UT, then there is your easy answer. If you don't want to do a lot of hacking around and don't have UT, there is no easy answer. If you DO want to hack around, then please tell me and I'll try to find or remember where the instructions on cheating are.
Blizzard.