Where normally, people see if they have it on themselves or not, for those three classes, one of the buffs should change.

Mage - Intellect.
Priest - Stamina.
Warrior - Attack Power.

For these specific classes, their specific buffs should show a small '10/14' number with it, to show how many people in the raid are buffed with their specific buff. So that people do not have to ask that often anymore, as they can see themselves, when some one does not have the buff they should cast.