It is a little more difficult if you have the steel lines to the distibutor but probably the easiest way just to see if it is working is to disconnect the vaccum line and install a rubber hose. Take the distributor cap off and with the engine off suck on the end of the hose and put your tongue on over the end of the hose. The diaphram should hold vacuum. If it does not the diaphram is leaking. When you suck on the hose, look down at the distributor, the plate should rotate slightly as you apply suction to the hose. If it does this and the diaphram does not leak it is probably functioning properly. You put the distributor cap back on and start the engine. When you suck on the hose, the engine rpms should pick up. If you have a timing light you would be able to see the timing marks advance as you apply vacuum.
I assume you have a Load-a-Matic distributor which does not have a centrifugal advance mechanism. You may not need to disconnect the vacuum line to see if the advance is functioning. If you have a timing light, you can see if the timing marks advance as you increase the engine rpm which increases the engine vacuum. If you have the Load-a-matic and the timing advances with increasing engine rpm, then the distributor is likely functioning properly.
Doug
