There is currently no way to tell if someone is looking down their sights.

I think, maybe in ARMA4, this would be a very cool thing to add.

As well, it would interact with the 3D scopes (the corrected ones which superimpose over the original view rather than replacing it with a zoomed version) in that rightclick would bring up the 3D scope, and then pressing "+" would put the eye to the scope, bringing the scope image from ARMA2 as a more focused look down the sights.