What does the suction gauge actually show?
According to the diagram below from a TB200 pilots manual the gauge shows the difference between vacuum line and the ambient pressure:



The pressure difference between the ambient and the vacuum line drives the airflow which spins the gyros in you instruments. In the Socata design, there is an implicit assumption that the air filter is not blocked. If the filter would be blocked, the suction gauge would probably exceed the green area unless the vacuum relief valve opens before the red is reached. 

If your vacuum pump stops working, the dial will fall to its lower stop. If the air filter is blocked, and both the vacuum pump and the vacuum relief valve working properly, the dial should exceed 5 inHg.



Other designs are a bit more fault tolerant in the sense that they do not make any assumptions on the working of the filter or the relief valve. In the examples below (thanks to voretaq7) the suction gauge indicates the pressure difference over the instrument itself, which is a more robust presentation of how the system is performing.