How do wheelchair motors detect speed?

I'm putting together all of the pieces for my first robot. The wiring harness and (AX2550) motor controller are getting ends
crimped on them today.
I've been looking at the motor controller manual and thinking about speed control. I'm using a (Quickie G-424) wheelchair as the base. The standard (QTRONIX) motor controller on that can apparently sense the speed of the chair. (It does a *nice* job of speed control.) I don't know how it does it though.
Each motor has four cables. Two are for the motor and two are for the brake. I'm wondering if the brake lines report speed also. I suspect this because I notice that the controller can sense when I manually release the brakes.
Anyone have experience with this? Otherwise it looks like I'm going to have to find a 'scope...
Thank you.
--kyler
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