presure relieve valve
 

how can i check that the valve opened after a overpressurization?

What aircraft type? Some Boeings for example, the 767, have a red piece of rubber material that is exposed around the circumference of the valve if it has opened.

Rubber jungle ?

Over pressure
 

The B747 overpressure valves have clam shell outer doors that can be seen on the left forward fusalage below L2 door.
The valve its self will open at a diff. of 9.4 psid. and close as the diff.pressure falls, but the doors will remain open. They will have to be closed manually on the ground.

The rubber jungle will not happen with a high diff. which will give a low cabin altitude, but with a very low diff. which will give a high cabin alt.

B737-200.You cannot detect if a valve opened in flight visually. If the crew reported a sudden psi drop then it probably opened then closes when psi drops. I have seen pieces of insulation trapped by the valve when this happens. Also the valves are supposed to be function checked during a C check.