Testing susceptibility to peer pressure in an individual mind is very a interesting idea.

Unfortunately, we can't accurately test it _yet_. A huge amount of data and tests -along with massive improvements in the understanding of the human brain- would be required to make a complete snapshot, without which testing isn't possible. We can't eve be sure this theoretical test is possible.

This alone wouldn't prove that their hands are clean, just that they wouldn't be as much to blame as someone who didn't succumb to social pressures and had more of a choice in the matter.