How can you know the fps or 480p/576p/720p/1080p/1080i by looking at it?

[Gosh]

How do you know it is not 40 or 50?

From an analysis of GTA IV, it show that Xbox 360's avg framerate is 32, PS3 is 29.

Some of the people say that earlier console can easily run as 60 fps, is that true?

How about portable handheld? Is that a different method to measure them?