Actually, sometimes it doesn't tell you because they've killed the first boss or something and failed the rest. I've gone without any message saying "NEW" especially in HM primals. I was in an AK run where no one was "New to the dungeon" and at the 2nd boss we wiped. The guy said, "yeah I never made it up to this boss."

ESPECIALLY, when you ask in the beginning "everyone know the fight?" and there is silence. You mean to take the silence as an affirmation but some people are just too scared or something to say they are first timers. I have no problem with anyone wanting to learn as they go - but I'd love to know if we need to explain stuff before pulling.