Personally I'd be likely to do it this way, which is very much the way it happens in a movie or book.
Everything's a minion unless it's getting extra screen time/description.
If I chuck down 20 minis and then put one behind them on it's own, guess which are the minions?
If the players meet a mixed bag of orcs, the minions are the ones where I say "ok, you get hit for 3,5 and 7", and the soldiers/elites/whatevers are the ones where I say "his mighty blow glances off your armor"
If you have to ask me which foe damaged you, then the one that did it is a minion.
That may of course work in reverse - a minion who crits the party leader as his first attack will quite possibly be upgraded to non-minionhood, while his buddy goes down a rank to balance it out.
Same goes for if people pick on a particular miniature. If my band of orc minis happens to have one in them that players call out as something other than a position ("the one with the yellow bandana"), he's probably not a minion.