Upvote:0
It depends if it's the appropriate time and the appropriate place and the appropriate intention.
Red flags? Like the teacher smoking cigarettes or the teacher who drinks regularly and calls it crazy wisdom or the teacher who only knows the Dhamma through just studying books and no experience? It is possible that an unexperienced teacher could lead one to liberation while missing out on it themselves, yes?
Upvote:1
"Preaching" in the sense of "proselytizing" is kind of unpopular among Buddhists (see also How are Buddhists supposed to spread Buddhism?). In the suttas I think that the Buddha tended to teach people who asked him a question or people who challenged him (and didn't try to impose his views on strangers), and he often taught by asking questions.
But "preaching" in the sense of "giving dhamma talks" is normal; and (Dhamma) is I think defined as one of the proper topics of speech for a monk.