To be save: It means, once I posted an article with steemit.io and it's published I cannot edit it, right?

That is correct, the ability to edit a post is not yet enabled. However we are getting there!

Also the question could rise how a user knows if he is whitelisted or not. Is there an opportunity to look this up?

No, but a database listing all white-listers on a separate tab on steemstem.io sounds like a great idea. For instance you are white-listed (high quality psychology content) but perhaps you don't even know!