You are correct in your thought that FreeCAD has a switch for making internal/external gears. My first time making a gear came out as an internal when I intended external and it took a while to discover that switch at which point it turned into an external.p.s. I managed to make my Ring gear. I had to create a "new body" first, then start the involute gear operation, then pad it.
Then I made a disc, and substracted the gear from the disc. There is an option external gear true/false. (I hope that makes internal gears).
How to test a ring gear? I think for what we do it is sufficient to just mount two of the small gears in the correct distance on some fixture and rotate the ring gear on them.
I just noticed that I spent hours to design a sprocket in another CAD thing, and that Freecad now has a macro for it.