Our perception of another's honesty is more likely based on our self-awareness than our instinctual knowledge of nonverbal cues. If I recognize that I would lie if I was in your situation, then I assume you would lie too.
Isn't this the crux of why we are unable to sympathize with some? Do we shun the inmate because we recognize the worst of ourself within him? Or because we have not explored ourself to the depth of realizing our capacity for immorality? We would like to believe we wouldn't 'snap' and commit a crime. But the truth is, were our situation different, were the time and place some other, we might. We might commit a crime that put us in prison. Are we each aware of that? Aware of our own humanity as we pace through our carefully planned days?
We cannot judge others for we have not been where they have walked. We do not know the nooks and crannies of their minds, hearts and pasts.