At what point do you say "I know Linux"?

Hi everyone. I'm new here.

I've been in a sysadmin job for 2 years. Our place is mostly a Windows shop, but I push Linux at every possible opportunity. The guys know me locally as the king of Powershell, since I prefer shell to GUI (Duh, it can be automated. GUI can't.)

Anyway, If I were to look for another job somewhere, I don't really feel confident in saying to an interviewer: "I know Linux", or that I have skills in Linux.

I can navigate a terminal, I can troubleshoot issues, I read manpages (omg!) But I don't feel like this is enough. So I refer to my post title: when did you decide you 'knew linux' ?

EDIT: Thanks for all the responses everyone. I have some ideas of where to go from here. Some gaps of knowledge that need to be filled, some new things to focus on, and actually going for some type of certification does sound like it will end in satisfaction. Cheers!