A friend of mine was re-learning tanking at the beginning of Cata.
His philosophy was go in, tank, try not to say anything, and at no point give off the impression that he'd never tanked before Cata.
Basically, it's a confidence thing. If you act confident and you focus on your task, you're probably better off than asking questions of people.
By exuding confidence (and a bit of luck in that he never screwed up) he had people seriously impressed by the end of the run.

That was my philosophy when BC rolled around and I made the switch to healing full time. If people don't figure out that I'm new to this, I'm doing something right.

I'd say do exactly what they apparently expect of you. Go in, pull mobs at your pace, hold aggro, next pack, repeat. Bosses on normal are pretty basic, there's the dungeon journal to tell you what they do, and your role as tanking doesn't change dramatically in any of them.

If you do have to ask a question, I find saying the words "Stupid question, ..." rather than assume anything. I find that tends to get people in the right frame of mind to answer a question decently rather than not.