Alright, let’s start with an easy post. Did you know that the game tells you whether your units are spotted or not? If a unit is in cover and hidden from the enemy, the name tag will be animated, fading out and becoming solid again. If the unit is visible however, the name tag will be solid and not fade out:
There is no such indicator for units sitting out in the open, so you have to take a guess. But if your AA is sitting serenely in a forest with its name tag in solid color, that’s your warning to move it or lose it to oddly accurate mortar fire.