It's certainly possible that intelligence agencies are mistaken or deliberately lying. We can't possibly know if that's the best interpretation of the evidence

It's certainly possible that intelligence agencies are mistaken or deliberately lying. We can't possibly know if that's the best interpretation of the evidence they may or may not have. They have been correct in the past as well. The best and only thing the average citizen can really do is try to put people in office who can intelligently parse classified information and evidence from intelligence agencies.