I usually AM the newbie, rather than finding newbies XD In endgame content, anyway. I only recently (as in earlier this year) started seriously doing dungeons, and only on my hunter for that, so for a while I was very cautious and apologetic whenever anything messed up because I was pretty sure it was my fault.
Over time I'm starting to learn how to actually identify whose fault problems are. Sometimes it really is me, and sometimes it's the other dps or the healer or the tank. And sometimes it's still hard to tell...like the run I did the other day, when the tank kept charging in like crazy and we wiped twice; at first I thought he was just a dumb tank, until I realized at the end our pally healer wasn't even throwing Beacon up or anything. It just takes practice I guess.
I can say, however, I respond extremely well to patient help and extremely poorly to the rage response. I still remember a few months ago when Kalli, Fworb, Veph and I ran a BC dungeon randomly together as a 4man, and I did it on my mage...who I've never run any dungeons with before, ever. They were extremely patient with me, explained the rules of the fights, and gave me tips on how to play the class better. 'Cause of that I was actually able to improve and take advantage of a new part of my mage's abilities that I didn't even realize I had before.
On the off chance that I DO meet a nice-but-clueless person, I try to help in a friendly and calm manner, because I've been where they are and I know how much it sucks. Usually it's just little things on the map; where do I get this quest, do you know where this mob is, can you direct me to this trainer 'cause I'm lost, where do you get that pet? If it's in a dungeon, I can usually answer questions about the fights, and I usually give people the benefit of the doubt the first time (hey guys, just remember that you can't stand in the purple goo, okay?

).
If they're nice they're usually willing to take advice, which is a good thing at least
~VelkynKarma