there are few things to look for with relation to a PCV, you want to check the grommet it sits in. if its hard and cracking, its needs to be changed and probably along with the PCV. you want to ensure a good seal so that oil pressure remains good and consistent.
To test the actual PCV, you can pop it out and try to blow through it. you might get a little taste of oil (i speak from experience) but it should only blow one way and not the other.
It needs to blow from valve cover to intake, not the other way around. If it does, change it.
If its hard to blow from valve cover to intake, change it.
If its passes the above test, you are good.
PCV can affect gas mileage and oil pressure.
You want a good functioning PCV so that the vapor generated internally can be used and combusted back through the top of the intake.
Obviously another way to check a bad pcv, is when you clean your MAF and see inside your intake manifold has spots of oil and such then your pcv is not doing its job to only get vapor through and not oil.
BUT also understand its not perfect, a fully functioning PCV can and still transfer bits of oil to the intake manifold which is okay as long as its not a large amount. After a while, all the vapor can carry small amounts of oil and it can accumulate over time.
I think i covered all the bases, so change it if you need to and if i remember correctly its only like a 10$ part.
.
