Custom tags are the way to go, and fairly easy to do.
Code:
local myES = select(8,UnitBuff(unit,"Earth Shield")) == 'player'
You'll need to figure out how to use that in a tag. Event to watch is UNIT_AURA. If you need the number of stacks and not just whether or not the buff is there, it's about the same deal with a full list of UnitBuff returns.