Oh, I love this question because it's basically the difference between someone who talks about marketing and someone who actually gets their hands dirty. From my experience, credibility is built on proof plus showing up without being a slimy sales bot.
Take the SEO crowd. You see a post that's like, "My top 10 tips for link building" - snooze. But then someone drops, "Here's the exact email template I used to turn a 404 page into a backlink magnet - and it only worked because I bribed the webmaster with cake." That's gold. Specific. Real. Doesn't have to be perfect, just honest.
PPC? Love when someone admits, "Spent £5k on a campaign that tanked because I forgot to exclude branded terms. Here's the exact dashboard screenshot and what I changed." That's way more useful than some guru showing a fake ROAS of 500x.
Content and AI marketing - same deal. Show the weird prompt that accidentally made a blog go viral, or the time you wrote a whole guide and realised the client's audience didn't exist. The messy middle is where the trust lives. Not the highlight reel.
I've watched a colleague build a rock-solid rep just by jumping into LinkedIn comments and answering questions without ever pitching a course. No "DM me for the secret sauce." Just solid, free value. That's rarer than it should be.
Three things the credible ones do: share real flops and wins, explain the why behind the process, and turn up consistently without treating every interaction as a lead gen. Honestly, if you're not showing the crap you cleaned up, I'm not buying the glamour shots.