Few things:

1. In the new dungeons, the first-timer message won't appear.
2. If you're new, say so. I'll take an extra 5 seconds to explain mechanics, and trash mobs aren't that hard, should be able to kill 'em easy even with a new healer/tank. Trash pulls are the same in every dungeon.
3. If you say nothing, I'll take an educated guess and say that it isn't your first rodeo. Haven't been wrong yet.
4. If you are on PS3/PS4, it's not hard to make a macro "First time!" "On PS3/4, no KB lol".
5. These are such minor things but they have a HUGE effect on how smooth the dungeon goes.
6. You can get a USB PS3/4 keyboard for under 20 (of your local currency). Take the plunge and get one. It will seriously make your life SO MUCH EASIER. =D