A local exterminator does smoke tests for the integrity of the plumbing stack: pump smoke in the outside drain, and see whether it all goes out through the roof

A local exterminator does smoke tests for the integrity of the plumbing stack: pump smoke in the outside drain, and see whether it all goes out through the roof vents, or some escapes within a house. That's how they found where rats were getting into our basement--a plumber had removed a vent pipe and had not secured the end.