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.
Be fearless and pure; never waver in your determination or your dedication to the spiritual life. Give freely. Be self-controlled, sincere, truthful, loving, and full of the desire to serve...Learn to be detached and to take joy in renunciation. Do not get angry or harm any living creature, but be compassionate and gentle; show good will to all. Cultivate vigor, patience, will, purity; avoid malice and pride. Then, you will achieve your destiny.
History with its flickering lamp stumbles along the trail of the past, trying to reconstruct its scenes, to revive its echoes, and kindle with pale gleams the passion of former days. What is the worth of all this? The only guide to a man is his conscience; the only shield to his memory is the rectitude and sincerity of his actions. It is very imprudent to walk through life without this shield, because we are so often mocked by the failure of our hopes and the upsetting of our calculations; but with this shield, however the fate may play, we march always in the ranks of honor.
Winston Churchill (1874 - 1965)
Source: Tribute to Neville Chamberlain, House of Commons, November 1940