I agree with the above comments in that if your damage is too low against the first boss you can generally see when a party can't clear the Wall. There's no need to be rude about it, however, you can give them a warning saying "Yeah, I'm not sure if we have enough damage" or something similar.

However I have been surprised a few times and had my expectations turned on my head. I've had groups where we were absolutely abysmal on the first boss, but then everyone seems to get into their "flow" and clears the rest of the dungeon perfectly.