A manometer is really just a pressure gauge, but it can reference two pressures to show the difference in pressure (pressure differential). You put one end of it before the resonator section and the other end after the resonator section. You would then go for a drive in the car and make a few passes at wide open throttle (WOT). You can see at what RPM the resonator section actually starts to create a pressure drop (power loss). The larger the pressure drop the more power you've lost. However, if there is no noticable pressure differential until say 5000 rpm, your not gaining anything by removing it until you go over 5000 rpm. If your driving to get the best mileage you shouldn't be anywhere near 5k rpm.
Here is a rough pic:
