Sorry for the delay in answering,
Each man has his own perception of "fair". What is fair to one person is not necessarily so for another. The "yardstick" for each of us (imo) is therefore based on our own idea of fair.
I, personally, would judge someone to be truthful or not if I thought they truly "believed" what they were saying or doing. It is really their idea of truth which I must judge.
But that is really only judging the person.
To know the truth for myself is quite another matter. I have narrowed it down to the three things I said before.
A man is born. He Lives. He dies. All that he surrounds himself with in those moments are his own truths.