What do you mean, how would they know this?
Whoever administers the injection hadn't even put it in him yet. There was no lethal concoction going into his veins yet. So of course when he began acting like he was hurting really bad and in agony, they would know he was faking.
As for the terror of "being killed," at least he knew it was coming and he had time to make his peace with G_d.
I imagine the baby he slaughtered had some terror going on, also. The way I see it when you do something that monstrous, terror is part of what you're buying. I have no sympathy for him whatsoever. (Other than, I hope he made his peace with G_d, for his own soul's sake.)