I always take a careful look at party members, even if they aren't new, to judge how much I should be pulling and how to handle encounters. What I hate is when I'm running a dungeon for the hundredth time, new player alert pops up, ranged dps says he is new, etc. When we get to a boss, I make sure to explain what he needs to do and look out for. Then we die to a mechanic that was the healer's responsibility, ie. Siren's voice debuff or dots from first boss of Satasha HM. The ilevel 120 healer says "it's not my fault, I'm new".

So yeah, I prefer if everyone says up front if they are new.