When I was working in the dealership we would clamp off the pcv hose and pull off the vent that goes to the air filter housing at the valve cover. With that done put your thumb over the hole in the valve cover if it sucked your thumb in the intake gasket is blown if after a couple seconds you take your thumb off and it blows air out its building crankcase pressure and the gasket is good. Its about a 2 minute check when the engine is warm. But like said above if its bad enough you should see an abnormal amount of oil residue if you open the throttle and look in. For the vents switching to defrost when you are opening the throttle it could be the check valve in the vacuum line up by where the heater hoses go in to the core those went bad often. Check the intake first. If I had to bet I'm guessing the intake gaskets are bad if they are replace them and go from there.