Tell tale signs of a fatal double lung hit. Wag of the tail.
Really the only thing everyone should need to know is the hunch of a gut or liver shot. you see it hunched up, walking-off of a deer, you slip out and go home and don't even think about tracking that deer for a min of 12 hours. A mature buck during the rut 15+ hours.
If you can find the arrow, that will tell the tale if you didn't see the arrow hit. one problem, many fail to tell the difference between liver and lungs. Don't screw that up or you will bump a liver hit deer and the chances of finding it after bumping a few times is little to none.
I personally think after seeing, and being on MANY MANY blood trails, hunters REALLY shy away from that shoulder, and end up with liver shots. IMO if you're behind the shoulder and go rib-rib=dead deer. After I pull the trigger on my release, my TOTAL FOCUS goes into marking LANDMARKS (trees, a bush) wherever the LAST PLACE I SAW THE DEER!