Way to pinpoint when entire cell is loaded and "GameMode" commences for real?

[Asterra]

I've been taking it for granted that these things were working the way their names imply. But it turns out that a frame or two of "GameMode" can commence even before a game has finished loading, followed by a long span of "menumode" as the game finishes loading. HasLoaded3D does not seem usable in figuring this out, as it's happy to report that the player has loaded 3D, long before the whole cell has loaded and the player is given control. And as I've already noted, simply watching for "GameMode" is not terribly useful since the game seems happy to allow some GameMode during non-GameMode moments.