There is no check valve on the fresh air side because there doesn't need to be one. And the PCV is not really a check valve, it is more of a variable valve. It should never fully close unless there is a backfire in the intake. Let's go over how this works:
Under partial throttle, the valve is opened more (less vacuum to pull the valve closed) so crankcase pressure and the fact that there is more vacuum on the PCV valve side causes any blow-by and moisture caused by combustion to go out the PCV valve into the intake manifold.
At WOT, there is almost no chance that some vapor will escape out the fresh air side since the valve is now fully open (0 vacuum pulling on the valve) and manifold pressure is just below atmo pressure (contrary to belief, at WOT the manifold pressure is just below atmo, since the engine is still drawing air in). Even though the pressures are still close, monifold pressure is still lower than atmo, and combined with crankcase pressure it will tend to flow out of the PCV valve. Vapor out of the fresh side won't happen unless you have a lot of blow-by. If you have lots of blow-by, the crankcase pressure would force air out both sides because the slight difference between atmo and manifold pressure isn't enough to overcome crankcase pressure. Residue build-up around the fresh-air side would indicate vapor, which would indicate excessive blow-by.
When you let off the throttle, the throttle closes and the vacuum pulls the valve to it's most closed point (which usually isn't fully closed). Since the fresh side is before the throttle plate, there is no way for it to draw vapor through the fresh side because there is no vacuum being created in the intake pipe. Since there is no combustion at closed throttle, there is not enough blow-by, and no vapor to collect.
Unless the system isn't working properly or your engine is in really poor condition or under boost, you should never see oil residue in the intake pipe or anywhere before the throttle plate, period.
I've run breathers on my last 4 cars and have yet to see vapor come out of the breather, I haven't had any problems with it smelling more, nor have I had problems with oily residue anywhere around the breather. Even on engines with excessive blow-by. Why? Because my PCV system works properly and all of the vapor goes out of the PCV valve like it's supposed to.
The only exception to this is on my boosted engine. There is so much blow-by under boost that it will force vapor out both sides, so it would force air out of the breather. I fixed this by hooking both sides to the catch can and tied the catch can in to the turbo inlet.
Nate
