By design, KiCAD doesn't behave that way. This behavior is either a "charming eccentricity" for those who don't want the visual clutter of seeing a mask cutout over every pad; or an "annoying quirk" for those who'd like to see every design detail displayed at each step of the design.
And, as I tried to point out above, the mask openings aren't even precisely defined until the footprint is placed within a board design.
I have never done a specific test layout for a newly-created footprint, but I believe some designers do exactly that. Usually when I create or modify a footprint, I have a target PCB layout close at hand and I can verify the new footprint in the exact layout I created it for. It probably sounds a little condescending (or perhaps even arrogant) but my accumulated experience with KiCAD and other layout programs gives me the skill and confidence to know how a footprint change will manifest itself on a board layout. As the old admonition says, "Practice, practice, and practice!"
(And, for the record, before I submit a set of Gerber files to a fabricator I sit down with my coffee and a trusted Gerber viewer and carefully inspect every square centimeter of the board, displaying each layer and combinations of layers, measuring things like clearance, setback, annulus, etc. With KiCAD it is especially important to look at the silkscreens, since they are completely ignored by the DRC algorithms.)