One way to tell if it's working or not is to do a direct check. Pump the brake a bit then start the motor, if it's functional you should feel the pedal go down when the engine starts. The other method is having the engine idling and depressing the brake, you'll get a slight change in the idle as the manifold draws air from the HydroVac.

The HydroVac has a bypass valve in it so that if you have no vacum pressure or the engine dies you can still use the brakes. It is best to adjust the shoes out as much as possible, I can stop my truck with a dead pedal no problem (I did rebuild the entire system tho, so your results may vary).

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P