You'll need encoders on each wheel; you can probably get by with a simple pulse encoder (a disk on the wheel with alternating black/white sections, and an IRLED/Phototransistor pair to detect light/dark changes facing the pattern, so as the wheel turns, the pulse output is read by the microcontroller). You might also want to check into quadrature encoders on the wheels, but it probably isn't necessary for your application. At any rate, as the wheels rotate, you monitor the pulses - so many pulses on the wheels will equal 90 degrees of turning. It won't be perfect (wheel slippage), but it will likely be close enough (maybe close enough for the error to be almost unmeasurable).