As a DIYer how do you test your plumbing when you added pipes to existing plumbing?
Typically a &quot;test tee&quot; is installed at the lowest point possible in the main waste line. Then a test ball, an inflatable heavy duty balloon with a chain attached is inflated to block the pipe at that point. The vent is filled, many times from the roof, but in your case, you may have a place where the water can be added from the inside. I need to mention, the other drains are capped to contain all water to pressurize the whole vent/drain system.
As another mention, there is a test ball that is available that has the screw in plug with the balloon attached to it, so when the balloon is deflated the draining water is all contained in the pipe.