well there are actually a few different sizes :) and at lease one is a multi-function valve which would require two check valves .
I just bought a handful of them because they come in handy.
n80 is easy to check..
i used a 9.6 to 12V power supply with the bare wire (cut the end off) .
when there is no power you can not blow/suck anything through the. tight seal.
then you apply the power (don't worry about neg/positive you'll get it right). and it shold flow smoothly in both directions.
there's even a screwdriver to adjust it but i strongly suggest you don't do that unless you know what you are doing.
feel free to clean it. i tried everything on the planet but mine was jammed open a little and no matter what it was leaking air with no power; thus evap leak , mushy brakes, poor mpg, hard starts,etc.
there's a big pump in the back of the car that creates alot of vacuum. if you have the desire, replace the N80 with a straw (cut it down) and you'll see misfires galore and a sputtering engine. There's enough vacuum coming from the back of the car to stall the motor if the n80 malfunctioned and flowed 100%.
the n80 goes down the wheel well then back the rear of the car where this vacuum pump is located.
automatic cars have another pump of course in the engine bay. i assumed it was for brake assist or something like that.