Let me re-phrase.

How does a golfer know that an area marked by the Committee is an "officially" designated sensitive area vs. an incorrectly declared area?

Without some other piece of information, the casual golfer, AKA pretty much all of us here, can't know the difference.

I assume that for RCGA sanctioned events all of the course markings will be checked / corrected, but how about local tournaments?

The example that comes to mind is the Citizen tournament. Stonebridge #4 has a ESA down the left side. I have no idea if it is official or not.