Measuring the output speed with a high-speed camera.
You'll need a camera that can film at 1000 frames per second.
For example a Casio Exilim EX-FH100.
When recording, there should be plenty of light (otherwise you just get a useless dark shooting), and there must be a good background contrast.
Also make sure that there is a reference length in the picture, for example a meter stick or anything with a known length.
In the film below, the students have placed a piece of white cardboard immediately behind the firing plane. The height of the cardboard is 0.70 m and the boost pressure is 7.5 bar.
After recording your movie, put the movie into a video analysis program, for example. Tracker.
In the film below you can see the measurement of the potato projectile using Tracker.
As can be seen from the film the projectile speed is approximately 130 m/s = 470 km/h.
ATTENTION! In built-up areas (eg. In a school zone), you should only shoot at a wide, high wall when you shoot outdoors.
Do not aim at anyone or anything that may break, it's downright dangerous.
For indoor firing the cannon should be fastened and you should use a shooting guard (eg. a rug), see pictures below.
For indoor firing you should also only use foam grenades. Those you cut out of a chalkboard eraser and wrap with duct tape.
Wear protective helmet with visor and hearing protection.