I haven't really bothered to state it. Usually its obvious, "12 orcs rush out of the cave armed with scimitars and shields. They charge towards you. Behind them you can see several other figures emerging..." You hardly need to be told those 12 orcs are minions. They are filling the NARRATIVE role of 'cannon fodder', so that's what they are!
Now, once in a while a player has tried to guess that something or other was a minion and it wasn't, but that's OK. Some people say it "deprives the players of tactical information", but I disagree. Gathering that sort of information (by observation, making probing attacks, scouting ahead, capturing an enemy and interrogating him, etc.) are all pretty much smart things to do and part and parcel of any sort of action, just like swinging swords and such. I don't want to turn 4e into a detailed operational wargame, particularly, but I see nothing wrong with factoring into play some degree of learning about your opponents and adjusting your tactics. Its fun.