Normally if you queue for a dungeon and then leave immediately upon zoning in, you get a 30 minute debuff that prevents you from re-queuing for the duration.

However, if you try and enter the dungeon while in any quest vehicle (the moose you have to return to the Thunder Totem stable master, for example) you aren't ported inside.

You can then mouse over your group mates and see which dungeon you got. If you don't like it, you leave group and no debuff! You then only have to wait 15 minutes to re-queue due to the timer in the dungeon finder window.
TL;DR:

Dungeon popsBe in quest vehicleAccept dungeon queueMouse over groups mates, determine which instanceLeave if you don't like itFind another group in 15 minutes instead of 30

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