The same way you time anything with a camera. Take a video with a high frame rate camera. count the number of frames it takes to complete one revolution. Multiply by the fps of the recording.
In your case if it were exactly 1200 RPM and you were shooting at 60 FPS it would take 3 frames.
1r/3f * 60f/s = 20r/s 20r/s * 60s/m = 1200r/m
You really need a high speed camera to actually be accurate.
Thanks, i just needed some sense full answer, that is exactly what i got from you.