There isn't a callback for people adjusting their sights. However, what you can do is set up a tick that compares the player's eyeVector.
Something like..
if(%eyeVector $= %client.oldEyeVector) {
eyeMoved(vectorDist(%eyeVector,%client.oldEyeVector));
}
%client.oldEyeVector = %eyeVector;
in a loop.