The main hallmarks of critical reading are really understanding what you are reading. Not just skimming and getting the gist of it, I mean understanding what the writer is trying to get across. When you just read a text, you are doing just that, reading it; when you are reading it critically, that's when you can really understand it. I would say the difference between regular listening and critical listening is the same as the reading. Really understanding it. A person can listen critically and have any opinion, just because you are listening critically doesn't mean you have to agree with what is being said.