Some of this will just be the adult teaching the younster his place in the world, ha ha. I usually go by the sounds, but factor in the personality. Chestnut hollers up a storm all the time, so its tough to decide if he needs me to intervene on his behalf or not. With the others, the sound changes when it gets serious.

Keep a water spray bottle handy and intervene when necessary. (I know, your question was WHEN is it necessary.) You'll get used to the sights and sounds enough to know for yours, just as moms do for their kids.