When thinking about explanations for my feelings or beliefs:

"Is that a cause, or a justification?"

It's usually important to consider both kinds of explanations. Mistaking one for the other is an especially nasty trap -- rationalization on one hand, miscalibration on the other.

Examples:

Say I'm explaining to someone why I always take a particular bike route to work -- "it's less stressful to cross the major streets this way." OK, that's a justification. What might be a cause? Well, it could be that my brain evaluated alternative routes, considered all arguments, and came to the conclusion that this was the best route. On the other hand, maybe when I ask myself this question, I remember that when I started biking, this was the only feasible route due to construction. So maybe there's a causal factor here unrelated to the optimality of the route -- I haven't changed routes out of habit. That's a red flag that I might be rationalizing, so I start more seriously considering alternate routes...

Or say I'm anxious about a talk I'm preparing. I ask myself why I'm anxious, and my brain comes back with "every time you think about it, you vividly recall how someone asked a question you couldn't answer at your last talk." That sounds more like a cause -- and on reflection, it doesn't justify being so anxious. (That realization doesn't immediately cure the anxiety, of course, but it gives me a lever to recalibrate it. I'll also put more thought into what questions might come up this time.)