This is a great post from Reuban Saltzman, a very thorough home inspector. I've never had anyone check this and can see how it could be a big problem with kids taking a bath. Worth the water to fill up the tub and be sure to prevent drywall damage. Home inspections usually take 3 hours for single family homes. It seems there are always more things that can be checked though! You have to remember when buying a house that things do happen and inspectors can't possibly catch everything though.
Many years ago, I learned about a bath tub leak that I never caught during my home inspection. The seller didn't have any children and didn't take baths, so they never knew about the problem with the bath tub drain. I did my standard inspection of the bath tub, which included filling the tub up with about four inches of water and then letting it drain. I didn't find any leaks and never reported a problem.
After the new owners moved in, the first time their children used the bath tub water began leaking through the kitchen ceiling.
Why didn't I catch that leak?
Because I didn't test the overflow. The first time the kids took a bath, they filled the water up as high as it would go. When the water hit the overflow, it leaked right through the overflow in to the wall cavity because the overflow wasn't connected properly. If the overflow drain at the bath tub isn't connected properly or has a worn out seal, it's going to leak, and sometimes profusely.
That was a real 'duh' moment for me. I had never heard of other home inspectors testing the overflow, so I just assumed that doing this was beyond our standards of practice. After hearing about the leak, I realized that this was something that I could have been testing all along, as long as I can view the bath tub drain via an access panel in the wall or floor. I checked my Standards of Practice, and according to 6.1, A, 1, I'm supposed to inspect the plumbing fixtures. What that means and how I do it is pretty much up to me.
From that point on, I started testing bath tub overflow drains, and I've since found dozens and dozens of bath tub drains that leak at the overflow. I've also found that this test forces me to run a lot of water down the drains, and if the main building drain has a clog, there's a good chance that I'll find out about it after running all this water.
The video below shows the worst overflow leak I've ever found; this was at a house that was being 'flipped'. Luckily it was an unfinished basement, so the leaking water didn't do any damage.
If you want to test your own bath tub overflow, it's very easy to do. Just fill up your bath tub with water and watch the back side of the overflow when the water starts draining in to it. If it's not working right, you'll know.