Jim,

If the distributor moves when the engine is brought off idle (in the driveway with hood up) then the vacuum advance unit is working. It has a diaphragm in it that can fail and leak. This usually means it won't move the distributor and it creates a vacuum leak.

If the vacuum advance is doing its thing in the driveway, its doing its thing on the road.

If you wanted to check it..for grins..you merely put it under vacuum and see if it holds. The easiest method is to suck on it and plug it with your tongue..you will feel it if it holds.

Stuart