That should be designed to hold the same amount of pressure as a radiator if it is the type with a radiator cap on it.

And with that said, they make radiator pressure testing tools which have a pressure range of up to 30 psi.

Other than that, with things which hold pressure, there is...

-The pressure they typically work at.
-The "rated maximum pressure" (higher).
-And then the pressure they will burst at during testing (even higher).