Evo, that's where a warning light system comes in handy. In the late model 172S POH that I have, the system has 2 engine driven vacuum pumps. Each pump has it's own vacuum sensor upstream of the tee manifold. When either pump fails, a light (L VAC or R VAC) blinks for 10 seconds, then steady on after that. The light lets you know immediately that there's a pump failure.
Then of course there's the vacuum gauge to go along with the warning lights.