My neighbor convinced me that his plumber friend is the best. The two of them installed abs pipes in a concrete slab before a pour. No way to change things now because the pipes are in the slab. They created a wet vent. A 2" abs pipe is shared by both the vent and the water runoff from a shower. I am worried because the slope is not very steep and water stays in the pipe. Are there any ways to test if air is getting through to the vent? I'm thinking of like connecting a vacuum cleaner up to the pipe and sealing it with duct tape and checking if air is getting out of the vent up on the roof. Even if air is getting out I'm worried that maybe the opening might not be big enough. Roughly 2/3 of the 2" abs pipe is filled constantly with water that just sits there and won't drain due to the slope. It eventually leads to a 4" x 2" wye at a junction down the line that then goes out to the septic tank.