I have killed a few dozen deer over the years with a bow and in addition to seeing the arrow hit I always hear the impact. The sound of a well placed broadhead going through the heart lung area of a deer makes a definite smack as opposed to a miss. In all these years between seeing the arrow and the sounds it make I never had a question of a hit or miss. Like SH said the way the deer reacts after the shot is another hint as to what happened. After a good hit the deer often makes it a head low death gallop rather than a bounding tail up normal flight away from danger.