won't tell you if the cap is bad becuase you have to take it off to pressure test the system.

 

Every pressure tester has a adapter to pressure test the cap with the same pump that pressurizes the radiator. It's all part of pressuring the system the cap is part of the system

 

I would just make sure its not overfilled and get a new cap and throw it on and see what happens. It will probably cost you more to have it tested if you don't have a tester. just make sure the new cap is the same pressure rating( its maked on top of cap)