How new are you to tanking?
Honestly, this kind of sixth sense is something you build over time, you get a feel for how much of a threat each mob is to you and pull accordingly - Usually in endgame dungeons its made a little simpler by there always being a consistent 2-3 groups of mobs between bosses, so you can pretty much pull everything with proper cooldown management.

Usually if i end up in a lower level dungeon, and the tank is pretty new and pulling as small as they possibly can i will go and stand a bit further down and just...stare at the next pack of mobs, and i stand there for several moments...If the tank shows no signs of budging, i'll sigh and just buckle in to spend the extra 5 mins or so doing smaller pulls.

You'll get a better idea of what you can pull and where - But i always say, its better to go big or go home. I think its best to pull as much as you THINK you can, and if you wipe? Oh well, you wiped a single time, so just cut down on the mobs slightly until you can do it comfortably, instead of just being a scaredycat and pulling as small as possible in every single dungeon. Sometimes you just gotta take risks and play agressively to get better at your job :P