There's not really signs it bad, but a worn out one will allow much more oil into the plenum than a normal PCV will. Let's say this, my car had 88K miles on it (3.2L), threw a rod and got a used motor. The newer motor plenum was in bad shape, mine was fine, OK swap plenum's. I stood my old one up on throttle body and the next day the amount of oil that drained out was a big surprised. PCV valve still had some 'rattle' to it, a new one was much louder rattle wise, hence less clogged. So every 2 years or so???
70mm TB, True Dual Exhaust, 3:89 Gears, 300M Special Front and Rear Sway Bars