The symptoms of a bad PCV valve include rough idling, oil in your intake manifold, oil through spark plug gaskets of tappet cover or oil inside your PCV valve itself etc. Because PCV's job is to provide "Positive CrankCase Ventilation" by sucking the water vapor + piston blow-by gases out of the engine, if it is clogged, your car won't run well at idle. Especially when its warm. In my case, I had oil in my intake manifold as well as a leaking tappet cover gasket. PCV valves must be inspected regularly i.e., let's say, every 5 thousand miles or so. If you see oil inside the PCV valve pipe and intake manifold, or you simply pull the valve out and shake it near your ear and you don't hear it rattling, its time to get a new valve. It will dramatically increase your engine's power if you go from a bad to good valve. Personally I will check it every thousand miles or so on my car.