Never heard the term "check vent" before. After a quick stop at Google, the only pictures I could come up with look similar to an air admittance valve. Any more information Jim? Next, I must be blind 'cause I don't see a check vent or air admittance valve. I see an S trap and what looks like a sanitary tee (I presume it goes off to a vent stack maybe?). It looks like the tail piece has some writing on it but I can't make it out. If the tail piece is the valve itself then they do need to be installed in the vertical. Also, they can't be buried behind a wall as they might need to be replaced/serviced in the future. Thread drift alert:: Any freeware out there where you can right click on a picture and then zoom in?