Perhaps there is no other knowing than the mere competence of the act. If at the heart of one's being, there is no self to which one ought to be true, then sincerity is simply nerve; it lies in the unabashed vigor of the pretense. But pretense is only pretense when it is assumed that the act is not true to the agent. Find the agent.
It is so little true that martyrs offer any support to the truth of a cause that I am inclined to deny that any martyr has ever had anything to do with the truth at all.
We should not be so taken up in the search for truth, as to neglect the needful duties of active life; for it is only action that gives a true value and commendation to virtue.
We are always hearing of people who are around seeking after the Truth. I have never seen a (permanent) specimen. I think he has never lived. But I have seen several entirely sincere people who thought they were (permanent) Seekers after the Truth. They sought diligently, persistently, carefully, cautiously, profoundly, with perfect honesty and nicely adjusted judgment- until they believed that without doubt or question they had found the Truth. That was the end of the search. The man spent the rest of his hunting up shingles wherewith to protect his Truth from the weather.