IF the blades opened, even on soft flesh, there will be DENTS in the arrow's insert that line up with the blade groove from the force of the blades snapping open under force.......they probably won't be as extreme as that jammed blade cutting the arrow shaft itself, but there has to be marks in the insert if the washer wasn't used.

2 definite ways to tell if only one blade opened.........check the EXIT wound and the organs, or if you forgot.....check the arrow's insert for small grooves cut into the edge.