I couldn't, and I don't believe the human eye is physically capable of discerning the difference between 40fps, 60fps, and 90fps.
I'm pretty sure that without motion blur (and I think sswift will confirm this), you can distinguish much higher frame rates than 30 FPS since the lack of blur makes it possible to notice the individual steps between each frame.
Personally, I can tell the difference between 30 FPS, 60 FPS, and 90 FPS very easily (with no motion blur). But then again I can also tell between different frame rates of TV programs (and that's with natural motion blur, of course), so maybe I'm unusual. I usually prefer 60 - 75 FPS steadily if possible for the most smooth gameplay. 30 FPS is acceptable, but can make your eyes tired if you watch it too long.