Implementing Traction Control for an advantage in the 2009 game

[lemon1324]

Would the "pseudo-pulse" be variable depending on conditions? If not, then you're effectively driving the motors slower, not actually preventing a slip.

A Traction Control System looks at the all of the wheel speeds and determines which are slipping, then slows only those wheels.

My team is definitely trying to do this [4WD, all independently driven] but we can't figure out one thing:

How do you determine if a wheel is slipping when all four wheels are powered?

Any answer would be appreciated.