If you have a Fantastic or similar type fan it is easy to check vent.
First off go on the roof and tape down one side of a piece of paper or light plastic so that it lays over the vent. Next close all windows and vents. Turn on fan to high blowing IN. With someone outside watching the vent open the toilet valve. If the vent is good the person outside will see the piece of whatever blowing. If you keep allowing the water to flow in the tank the it will shut the vent off as it reaches the pipe. At this point it will be noticeable that the vent is no longer blowing air and you will probably notice the aroma. Now you know at what level the vent is into the tank.
Art.