There are two ways to check for spaces. The one that's usually sufficient is to stack the rings, hold them up to eye level, and look for any light coming through. Occasionally a bulb or flashlight is needed, but that's no big deal.
The second method, which is easier and may actually be more reliable, is to see if a sheet of paper can be pushed through the rings at any point. The rings in the photo passed the light test, and it was only a matter of luck that I decided to use the paper, as well. What a surprise! It was actually a quick fix using the old reliable sanding tile, and will result in a much more uniform effect once the interior is sanded.
When you've already put a lot of effort into a bowl, don't cut corners just as the end is in sight!
