Whenever a bullet strikes your intended target (like deer or elk), if carefully observed, you can often tell you if you hit your mark or not. Note: These reactions are general in nature and all animals do not act the same (i.e. depending if they're calm, on alert, just breathed in, or the cycle of the heart pumping blood).
Heart or Lung Shot
Upon being hit in the heart or lung area, most deer will usually jump or bound forward - kicking out forcefully with their hind legs. This shot produces a bright red frothy blood trail with pink or white flecks of lung tissue in it. Blood from the mouth is an obvious indication of a shot to the lungs.
Reactions include running a short distance with its head high or well forward before dying within 100 yards. Blood trails tends to be very dark red / thick and glutinous. No animal can survive a (too far behind & high) shot to the liver, whether or not the hunter finds it.
Stomach or Gut Shot
Gut-shot deer usually hunch-up and stagger away into nearby cover with their head held low. This poor shot (that is too far behind the vital zone) usually produces a lot of green-like splashes of rumen from the stomach - content that sometimes has pines, acorns, or hair, but with very little blood.