When you die, assuming you have no companion, team mate, or quest follower still there fighting the minions, the minions will cease fighting and group up and their timer will start. The minions will very subtlety flash, about once a second for roughly 10 seconds? (I'll have to get back to you with an exact answer on that..)
If you have a quest follower though, they will some times stay and continue fighting the mobs, keeping them from resetting and beginning their heal timer. This can screw you with their enrage timer though, if you take to long they enrage doing more damage and they put a debuff on you dealing damage over time.
Addition: I've been farming inferno and noted that some of the elite spawns that don't group up tightly, like the flying bugs, don't heal properly, we came back after a minute + and they were still at the same hp level. While this was helpful, I can see how it adds confusion to the mobs healing characteristics.