ActionScripter (Josh), Lead Developer, said this about in-game RNG:

"Many missions have it, some do not. There are randomizer modules placed on various things, like whole platoons and events in the mission file. Often there will be, say, 3 different choices for something, like where a group of enemies is coming from. The system rolls the dice for each module and picks one of the possible choices in each.

We used to have small die icons on the missions that included randomization, but this was removed when we re-did the menu.

I do not have a list of all missions that feature randomization, as it's a tool used by the mission makers at will, and they don't really report on it."