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.
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.
But say, my lord, it were not regist'red,
Methinks the truth should live from age to age,
As 'twere retailed to all posterity,
Even to the general all-ending day.
Every discourse is an approximate answer: but it is of small consequence, that we do not get it into verbs and nouns, whilst it abides for contemplation forever.
I believe it is an established maxim in morals that he who makes an assertion without knowing whether it is true or false, is guilty of falsehood; and the accidental truth of the assertion, does not justify or excuse him.
Our intelligence is imperfect, surely, and newly arisen; the ease with which it can be sweet-talked, overwhelmed, or subverted by other hardwired propensities - sometimes themselves disguised as the cool light of reason - is worrisome.