Couple of points here...
I think people are confusing "Minion" as a word, defined roughly as an underling who merely answers to the real people, and "Minion" as a mechanic of the game, meaning only what is stated in the articles thus far.
There is only a couple differences between a minion and a real mob so far as I have seen: 1 HP and 1/4 the XP. They are equipped the same, hit the same, move the same, etc...
In that case, if it looks like a duck, quacks like a duck, waddles like a duck... must be a duck! Is gonna be the players' general beliefs I think. Consider the following situation that could be entirely too fun to throw at some players:
The players are infiltrating a base of spies/assassins, and know that they utilize only the best monsters/NPCs to accomplish their goals... They accidentally trigger one of the silent "message-type" alarms and are ambushed by 20 humanoid creatures, which after one dies, the players, through knowledge check, discover that all 20 are dopplegangers. At this point, they know that at least 1 was a minion, he went down too easy... but these are dopplegangers... which others are minions, and which are not?