I don't think that is really possible. If you are in a quiet gym and no one has pepper sprayed you or punched you in the face, you can notice the smallest change in the person you are choking, or their little one finger tap.
If someone punched you in the face, if people are screaming, if you and the person you are choking are covered in pepper spray, then then cop will not have any reliable way of knowing when to let up on the choke. I think choking someone is high risk, high reward. Sometimes you apprehend the guy without hurting him (though cops want to maim and injure people so this is besides the point) and sometimes you press an attack against a helpless person and commit murder. Cops who apply a choke should use it to gain a position of control. Choking people out should be considered lethal force, and if the cop persists in a choke after the person is out, then it should be first degree murder / attempted murder, and the cop should be executed by the state. I'd swing the axe myself if they would let me.