I have noticed while playing that followers tend to say something like "Look at the size of that beast" or things of that nature when a unique/champion/elite monster is near. Is this dialog triggered 100% of the time, or only sometimes? I'm interested in using this as an aid later in the game when trying to maintain the Nephalem Valor buff - when it is critical to be hacking down every unique/champion/elite monster in sight (including those in sight of my follower).
It's triggered almost all the time you encounter a Rare/Champion pack is visible. I say almost all the time because I have noticed instances where my follower has not announced them. I am yet to determine whether that is due to an internal cooldown from him talking previously or if he simply does not announce it 100% of the time.
I have also noticed a single time where he used one of his sayings usually reserved for after a fight with a Rare/Champion pack after a normal battle, as this threw me (I was not expecting it). However this was post-fight, so should not affect your concern.
In conclusion, their sayings are a good way to indicate a Rare/Champion pack is near, but their silence may not mean an absence of said packs.