How can one know that he is qualified to give a Sharia lesson?

Calling to Allah is one of the best deeds, as Allah the Almighty says: "And who is better in speech than one who invites to Allah and does righteousness and says, 'Indeed, I am of the Muslims.'" However, one should not undertake general da‘wah, such as delivering sermons and lectures, except for those who are qualified to do so. Whoever wishes to do so should present themselves to callers to Islam and scholars for them to assess their level of knowledge and guide them. If they testify to their qualification, then it is permissible for them to admonish people and teach them.