You must give it time to die. As much as 5 hours or over night on a bad hit like a gut shot. While you wait, try to recall everything that has happened. When you begin trailing, if you can't find blood, look for your arrow. Finding your arrow is the only sure way to tell if you got a hit or miss.
Many excited hunters find blood while looking for their arrow that was missed.
The arrow itself can tell you a lot about the hit. A bright red blood may indicate a heart or artery shot. A pinkish bubbly blood means a lung shot. Green and brown matter or food particles means a gut shot. A greasy tallow on the arrow could mean a brisket shot.