Outside the vehicle, you can connect the vacuum hose from the in-vehicle booster to the booster you want to test, or get a longer but similar type of hose to connect to the booster being tested.
With the hose connected, idle the engine and then push the rod in with your hand, just as if you were applying the brakes. Then check how the rod recovers. It should be an almost immediate smooth back out movement; otherwise, the diaphragm is sticking.
Then turn off the engine, and disconnect the vacuum hose. Wait a couple of minutes and remove the check valve from the booster. You should hear a hissing sound when the valve is removed. That tells you the booster chamber is holding a vacuum.