Sometimes yes, sometimes no.
I think for ease of play concerns and player tactics, the default example should be "yes". Knowing which opponents are minions allows players to be more confident in their tactics and will result in battles being run in a different way.
However if the DM wants to run a "gotcha" encounter where the players aren't sure which creatures are minions, he's perfectly justified in doing so and let them puzzle it out.
I think for ease of play concerns and player tactics, the default example should be "yes". Knowing which opponents are minions allows players to be more confident in their tactics and will result in battles being run in a different way.
However if the DM wants to run a "gotcha" encounter where the players aren't sure which creatures are minions, he's perfectly justified in doing so and let them puzzle it out.