How do you find out if the vacuum advance(on the distributor) is working on an 83 chevy, 350 motor?

Malfunctioning vacuum advance shouldn't prevent starting unless the timing is already way off. Disconnect the hose and see if it starts (there shouldn't be any vacuum in the hose at idle because its either ported or switched electronically.) To check the vacuum advance, remove the distributor cap and connect a hand operated vacuum pump to the hose connector on the advance module. You should be able to see a rod or lever moving in the distributor as you apply vacuum.

Your problem likely is not a vacuum advance diaphram. Normally when what you describe occurs, it's the coil. When they get hot they don't put out enough spark. When they are cold they do. That should be an HEI distributor with the coil inside the top of the distributor. That's a starting point. I don't remember the specs, but you can ohm the coil to determine whether or not it is good/bad.

its not the vac advance, not sure what it is, but vac advance is only relevant while running.did you check to see if you have spark in the secondary(spark plugs and wires) ignition?ill bet if you do its weak.there are several ways to check vac adv.,as well as some neat tuning tricks.try a speed shop for custom weighting,youll love the acceleration.