You can test the pump for amplitude of vacuum but not volume. Anyplace you tee into the supply side of the system will give the amplitude (IF THERE ARE NO LEAKS). Thus the best test is at the first tee with everything else disconnected.
There should be no flow of vacuum through the booster unless the pedal is pushed, thus if you check the vacuum at the source tee with the line hooked to the booster it should give the same reading as if you remove the line and put your finger over it and retest. Same vacuum both cases then no booster leak inactive. Next step on the brakes. A large drop in vacuum will result but should stabilize and pressure build if the pedal is held with constant force (not pumped - pumping will continue to deplete and exhaust till there is no more reservoir of vacuum). Experience here is the only way to judge quantity of flow or pump capacity.
The main test for the booster is that the thing doesn't leak in either condition; only when pumped.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician