Glenn - I don't know..not exactly, but it should increase voltage with speed.

If you're driving along at 15 mph in a straight line, the output voltages from each sensor should be roughly the same (As Uncle Tony has posted above)

Need to figure out which pins on your ABS ECU connector are from the sensors. Instead of jamming the multimeter probes in the connector, which can spread them out and cause poor connection,, use a paper clip or similar. (Sorry if this is teaching you to suck eggs)

It's a lot easier on modern cars where you can just view live data via the OBD socket