Everyone has their own opinion. I have heard a ton of info from all over, everything from hind legs kicking in the air is a good shot, to deer getting low to the ground and running with tail between legs is a gut shot, and everything in between.
One of the first deer I ever shot was with a 30-30 at about 30 yards, standing broadside looking right at me. I shot and she just stood there. I thought I missed, I couldn't believe it. I slowly ejected the spent casing and loaded another one in. I raised the rifle to take a follow up shot, put the bead on her, just about to squeeze the trigger, but then... SHE FALLS OVER DEAD! I was like "What the hell was that?"
When I dressed her, it was a double lung shot. Go figure.