Testing encoders on the "bench"

Hello all,

I was given a bunch of Pittman motors (DC brush) that have built-in
encoders(500CPR)

The problem is some of them "might" have bad encoders, so how could I
test them on my work bench? Is there a simple circut I can build and
plug into the motor harness to check that the encoder is working
correctly?

something like, LEDs that'll turn on and off or speed up or slow down
when the motor is activated and speed up or slowed down?

thanks,