You’re quite welcome. As Ian said, in the web-based version, the only thing currently for determining if your model is solid is what Entity Info shows. He’s surely right that you’ve seen some people using Solid Inspector which is indeed very handy.
As much as possible, you can and should use modeling methods that naturally result solid shapes. Things like the internal faces in your model can be avoided, for example, by using a logical workflow. Also avoid creating stray edges which can create problems. For small things like those that are typically 3D printed, set the units to meters and work with them as if they were millimeters. STL files are unitless so it doesn’t matter that the model is drawn 1000 times larger than the real thing. You’ll just tell the slicer the units are millimeters.
Actually, the reason you didn’t see any holes when you tried WR’s file is he used (his likely default) Architectural units. When you exported his file, with model units and used millimeters for the slicer units, those holes would have a diameter of about 0.394 mm. Probably not printable.
FWIW, I noticed that some of the circles in your model were not drawn with the radius dragged out on axis. It’s a good idea to get into the habit of drawing circles by dragging out the radius on axis. It will result in better, easier to work with models. Especially when you have concentric circles or other geometry joining the circles.