I actually use passive Insight for this determination (unless someone calls for an active insight check to spot whether the foes ahead look like seasoned vets or nameless rabble.)
Basically, if the insight score is good enough (usually DC 10+level of the minions, I figure) then I might see fit to mention that the mob of (kobolds, orcs, whatever) up ahead look like a bunch of raw recruits, basic soldiers, a nameless rabble, whatever works to equate "minion" in terms of what the PCs are facing.
But as a DM, I prefer not to just state outright; it's gotta have an in-game reason, and some insight. In fact, one of the more entertaining recent encounters I ran was when the PCs, used to facing decrepit skeletons, ran up against 6 regular skeletons and a level 4 wight; they assumed it was a bunch of minions and one "important guy" and were alarmed when the skeletons not only didn't drop at the area attacks the wizard unleashed, but surround and started beating the snot out of the fighter. Good times! That kept them on their toes, afterwards....never just assume minions are on the board without good reason....heh heh!