I tend to agree that anyone who denies the tendency to rationalize is either in denial or has a different definition for the word "rationalize". In fact I would argue that rationalization is the default for human beings, and that anything else requires either focused effort or serious mental re-programming (which is still probably only partially effective).

One possible way to try to elicit an understanding for any given individual's capacity for rationalization is to ask them about the last time they did something they knew was a bad idea (perhaps a comrpomise they felt uncomfortable making, or an indulgence they knew they were going to regret), and then to ask them what excuses went through their brains to justify it. If someone still denies ever having had such an experience then they are beyond redemption.