How can i easy check if the value jumps from '-' to '+' ?
Pretty easy, just check if the most significant bit changed. If it did, the value switched from positive to negative or vice versa.
did you create the code yourself or do you use the physic engine?
This really doesn't matter as the physics engine changes the same parameters as eg. c_move would do (well, in this case it would be more c_rotate).
To solve ops problem: Just measure how long one rotation took in seconds and milliseconds (stored as floating point). When calculating the rpm, you can simply do this:
var rpm = 1.0/timeForRotation;
