Proving a person's state of mind is difficult at best, impossible most of the time.
Even reading letters, & diaries. More times than not, people write what they think people want to hear. Or worse, they write how they want to be perceived, not necessarily what they really think. There are many examples historically, all the way to present day. Today's equivalent of letters, & diaries, are twitter feeds, & FB postings. They aren't always a great insight into someone's real thoughts. There's plenty of motivations to be deceitful.
I tend to believe peoples actions, much more so than their words. What somebody actually does, is much better insight into who they really are, & what they actually believe vs. what they say. I suspect that has always been the case...