Assuming you are talking about a vacuum canister used in a vehicle:
The most important aspect of testing a vacuum canister is to ensure there are no leaks. To test for leaks, you will need a vacuum pump and a vacuum gauge.
Attach the vacuum pump to the canister, and then turn on the pump. The vacuum gauge should read between 18 and 22 inches of mercury. If the gauge reads below 18 inches, there is a leak in the canister.
To test the canister for proper operation, start the engine and let it idle. The vacuum gauge should read between 18 and 22 inches of mercury. If the gauge reads below 18 inches, the canister is not working properly.