How do you know if your thinking is unrealistic in cases where there is great uncertainty?

I had asked a question about Cognitive Behavior Therapy, and the discussion there made me wonder this. It seems that one tenet is that you should evaluate your source of anxiety rationally, so you can show yourself you are worrying about something that won’t or cannot happen.

Like if you are worried that your boss asked you to meet with him or her, and you think they want to fire you. You tell yourself that you have had good reviews every time you’ve been reviewed, and there’s been no rumor that your department is going to be cut. However there have been cutbacks and the company is not doing well.

Another example, I think, of a situation with great anxiety comes from questions we see a lot. “How do I know if he likes me?” This can be asked before you’ve dated someone or in the early stages of the relationship. You might wonder whether if you use the “L” word will he or she dump you?

You might become used to talking to your new love three or four times a day, and then one day, he or she misses a couple of the normal times, and you start getting anxious that they no longer love you and are using silence to show you you are no longer interested.

How can you know if that is unrealistic thinking? The relationship is new and you hardly know anything about the person. What would you tell yourself in that situation to calm yourself down?

Do you have other situations where you have a hard time knowing rational arguments to calm yourself down? How do you work through those situations?