If it's not a secret, how, exactly?
Short answer: Enough that I couldn't make it crash or malfunction horribly to being unplayable.
When I do quick checks of builds before uploading them, I go through all the base menus, try different game screens, try doing things in game like building a new base, installation, etc., buy/sell a few things, randomly research, produce, and equip items, and do this to the point of reaching a ground mission and moving a few soldiers and firing a few weapons. I also quickly test skirmish mode.
If the build doesn't have any noticeable bugs, crashes, or anything that makes it totally unplayable, with the exception of possibly a few minor (such as cosmetic) little glitches here and there, I stamp it with my seal of being "relatively stable."
Of course, that's never an absolute guarantee...
Please keep in mind that, at least as I understand it, development builds are mainly for bug-hunting and testing new features, not for entertainment/play value, which is what the last stable release is for (At the time of this post 2.2.1).