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.
Your bait of falsehood takes this carp of truth,
And thus do we of wisdom and of reach,
With windlasses and with assays of bias,
By indirections find directions out.
Even as wisdom often comes from the mouths of babes, so does it often come from the mouths of old people. The golden rule is to test everything in the light of reason and experience, no matter from where it comes.
Love does not fail for you when you are rejected, betrayed or apparently not Loved. Love fails for you when you reject, betray and do not love. Therefore .. Love.
You will find that the truth is often unpopular and the contest between agreeable fancy and disagreeable fact is unequal. For, in the vernacular, we Americans are suckers for good news.