Well, how did he determine that he dropped two powder charges in only two of those?
I've reloaded some shotgun over the years, about 20,000 rounds or so (but, almost all the exact same target load), and what strikes me is that in most modern loads using today's wads, if you double up on the powder charge I would think it would throw your ability to crimp the shell properly off quite a bit.
You might not even be able to get the crimp set because of the additional height of the powder makes the shot column go above the crimp line.
IF, however, he did overcharge one AND it took a normal crimp, there really wouldn't be any way of telling for sure...
Until the barrel on your shotgun blows wide open...
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.