If a garage is full, and the system is working correctly, the big signs on JFK and Will Clayton listing the airlines in each terminal will generally show that the garage is full. Look in the bottom left corner of the signs under the work "Parking."
Also, the garages at IAH do have open space counters on each floor, which tend to be pretty accurate. There are also signs with open space counts around the entry gates, to help you decide which floor to park on before starting up the ramp.