As per Redsaw - it's primarily a combination of factors. Not so much to do with knowledge (although it obviously helps!) but more how honest and helpful you are to others, how friendly you are, how many sensible posts you write, how many thank you's and likes you receive, etc, etc.
If you can have loads of knowledge but are condescending to "new" plumbers telling them they're idiots whenever they ask a question, you won't become trusted. If you are honest and friendly but keep giving wrong or dangerous information you won't become trusted. Pure quantity of posts won't work either. Nor will 5-10 quality posts. Creeping to the moderators, bribing, etc, won't work!
It's not a numbers game, but if you can obtain 10% thank you's (1 in 10 posts) and 20% likes (1 in 5) it won't take you long to become a trusted.