[RyanCahoon]

Welcome to the forum Take a look around, there's a lot of info here already.

While it depends on what kind of crab you're building, what you probably want is called an absolute encoder, which you can think of as like a potentiometer, except that it can twist forever, and outputs the angle from the zero position. My personally preferred brand when it comes to encoders is US Digital, but shop around. Connect it near one of your wheels, so that one rotation of the sensor equals one rotation of the wheels.

Alternatively, if you're using a non-coaxial design, i.e. your modules can only turn a certain number of times in one direction before your cables get wound up, you can use a multi-turn potentiometer, which will allow you to sense where in their entire rotational range your modules are.

Good luck,
--Ryan