Don't quote me, but I believe they are Hall effect sensors. Trace the wire in to the engine compartment, you will find a plug. There should be a reading of " x" ohms, and when you spin a he wheel that value should fluctuate.

Someone with a service manual can chime in with the specific readings.

If the abs turns on on the first press of the brake pedal I'd be looking for a broken wire, or something a bit more obvious.