start the car and pull the PCV valve out of the valve cover and put your finger over the end of it and see if you feel it sucking.
They don't go bad. Either it maintains vac or it does not. If not, just replace gaskets.
The valve cover on a typical engine is a non wearing part. If it were damaged it could be bent, warped or cracked and could be the source of an engine oil leak.
Check the compression of the cylinders you will have a low reading in the one with a bad valve
Depending on the year, make, model and engine, the engine may have a rough idle, poor performance,\ and check engine light illuminated.
If the pressure is to high or to low there is a problem.