It's in the delivery
The way to become an expert is to get a book published. This is what you'll find every publisher telling you when you start looking for one!
Perhaps it was more true in the past, when publishers took a risk and actually paid you an advance for the privilege of making money off your work. People believe having someone on the outside invest in you legitimizes what you publish.
Still, the perception remains; if you get something published, that means you have some expertise when all it's really saying is that you took the time to articulate something you know about. If what you say in print rings true, then people will flock to you. If you just sling BS, that will get picked up, too, and you'll die alone!
Ambulance Driver wrote a book about his experience in EMS. It rings true to most of us so beyond the fact he got published, he got believed. Then, he hit the blogosphere and made it quite clear he IS an expert in the field.
Rogue, although not having written a book has made his expertise clear in EMS by stating verifiable truths and backing them up with references and information.
But neither of them is a Know-it-all primarily because they don't jam their opinions down your throat. They state their cases as they see them, back them up with proof or illustrations and then challenge you to make up your own mind.
A Know-it-all doesn't leave anything up to you because he/she knows it all! So the difference, in my mind, is Know-it-alls show their stripes and lose the respect of rational and self-directed people. Experts are turned to for the wisdom they have gained from their experience.
How they present themselves and the information they've gathered over their lifetimes pretty much determines wheteher they're considered experts or know-it-alls.
Just to be clear, I'm not an expert in this, but I DO know a little!