Whenever a bullet strikes your intended target - like deer - if carefully observed, (9 times out of 10) its reaction will tell you if you hit your mark or not.
Heart or Lung Shot
Upon being hit in the heart or lung area, most deer will usually jump or bound forward - kicking out with their hind legs. This shot produces a bright red frothy blood trail with pink or white flecks of lung tissue in it.
No deer can survive a shot to the liver. 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.
Stomach or Gut Shot
Gut-shot deer usually hunch-up and stagger away into nearby cover with their head held low. This poor shot usually produces a lot of green-like splashes of rumen from the stomach - content that sometimes has a lot of pines, acorns, or hair, but with very little blood.