Whenever I start to get angry and defensive, that's a sign that I'm probably rationalizing.

If I notice, I try to remind myself that humans have a hard time changing their minds when angry. Then I try to take myself out of the situation and calm down. Only then do I try to start gathering evidence to see if I was right or wrong.

My source on 'anger makes changing your mind harder' was 'How to Win Friends and Influence People'. I have not been able to find a psychology experiment to back me up on that, but it has seemed to work out for me in real life. It suggests that, if you think someone else is rationalizing, then the first step to changing their mind is to get them to be calm. It also seems to suggest that calming yourself down and distancing yourself from whatever generated the rationalization (a fight, a peer group, your parents, etc.) is what you need to do to work through a possible rationalization.