You could try videotaping them in an argument and then go over the videotape looking for rationalizations. This could deal with varying definitions of rationalize. For best results, make the argument about something that people frequently rationalize. Maybe present them with some ambiguous data that might or might not support their political beliefs (several neutral observers say it didn't affect them either way, since it was so ambiguous), and see if it makes them more certain that their political beliefs are true (as you'd expect in a cognitively normal human).

I'm assuming you're using "rationalization" as a synonym for "motivated cognition".