Knowing when to speak and when to be silent is a difficult art, for as it is written in Al-Risalah:

Silence at the right time is the proper attribute for men, just as speaking on the right occasion is among the noblest qualities. The master Abu 'Ali ad-Daqqaq observed, "Whoever refrains from speaking the truth is a mute devil."

The only way we can know whether to speak or not, and what specifically to say, is through intuition. The Being can guide others, but first we must be guided. Therefore, we have to be very in tuned with our Innermost through remembrance of Him.

I always remain silent before people I know who are immature and have no open soil to receive the seed of these teachings, since if we cast our pearls before swine, they will turn again to rend us. However, I have met people who are very religious who, despite being rigid and fanatic, have opened themselves to the teachings precisely because of finding the appropriate moment in order to teach them. As to how one navigates such a difficult road, that depends upon remembrance of our Intimate Self!