Plumbing problems are common in homes, and while some of them might be easy to diagnose, others might pose a challenge. The rather easy plumbing problems can be easy to figure out and fix but this may not be so for the challenging ones. As the client did in this video, when posed with a rather difficult plumbing problem, the best way out is to get an expert plumber's opinion. This is because, from years' worth of experience, an expert plumber would most likely have the necessary skills and knowledge to diagnose and even propose a solution to the problem without having to waste so much time.
It's a plus if the plumber is local.
Together with your plumber, the first step to diagnosing a plumbing problem is to first turn off the water source and then proceed to inspect the drain pipe. You may want to test water outlets such as the toilets, faucets, sink, and water heater. Checking these water outlets would help you identify if the plumbing problem is from them or not and can help you clear any doubts rather than lean on assumptions.
Looking out for water stains on the floor, marks on ceilings, can also give you hints as to where the plumbing problem may be from.