What about the color of blood. If your not sure of the shot and didn't get a chance to see the exact location. Bright red blood is a good indication of a heart shot, frothy/bubbly blood is a good indication that it was a lung shot. Dark blood is a great indication of a liver shot.

I also like to leave an arrow at the last point of blood until I can find a new blood spot. This way you have a spot to return to and start over. It also helps to leave more arrows at the last few blood spots so you can line them up and get a direction the deer was heading.