On Sun, 21 Jul 2013 06:03:04 -0400, wrote:

Hence the question:
Q: How best can we tell whether a drain is working properly at any
one point in time?

Use food coloring in a small squeeze bottle. Squirt a little near the
drain and see where it goes. That is the same trick you use to find
pool leaks.

Third that vote. I was thinking food coloring before I read yours and
trader's comments.

Same thing for finding leaks from a toilet tank to the bowl.