Honestly, tell them you're new to the dungeon. Players will explain the mechanics and tell you to use LB.

Once you run the dungeon a couple of times, you'll get an idea of where players expect it (or not) and can use it at your own discretion. Players generally do not complain if you use the mage LB against a boss or a group of mobs (more than 1 pull or a bunch of adds with the boss).