How do you know if your exhaust brake is working?

When descending off Mt. Washington pass on highway 20 east bound where we have 6-8 miles of 6% down grade I nearly stop at the top, place my transmission in 3rd gear, put the brake switch in the on position and allow the coach to descend the hill, It will stay around 25 MPH all the way down. same thing on Highway 2 west bound, There I must use the airbrakes 3-4 times to stop the RPM from exceeding 2200RPM.