1. Evidence: Look to see whether or not you really have enough evidence to justify your statement. You will often find your "evidence" is rather weak and wouldn't be enough to convince you if you heard someone else say it. Zipping through your mind without examining it, the thought may pass. But write it down and look at it and you may at once have the horrifying realization that your brain is cluttered with bullpucky.
2. Alternatives: Often several factors influence the outcome of any given event. You may have latched onto the most demoralizing factor and decided that's what caused it. Look for alternative and equally likely influencing factors.
3. Implications: Even if you have plenty of evidence for your thought and it's the only explanation you can think of, what you think your explanation implies may be mistaken or unnecessarily self-defeating. For example, "Violence is the human condition." You may have plenty of evidence, but the implication is nothing can change it and that's a leap of fortune-telling and fatalism that has no place in rational thinking. By omission, it also implies that love and kindness are not also the human condition. The person thinking that violence is the human condition has experienced far more acts of kindness than acts of violence in his own life.
4. Usefulness: Sometimes you don't really know if a statement is true or false. For some statements, the question of true or false doesn't apply (for example, an overblown generalization such as, "society is evil" can't be rationally argued one way or the other without being ridiculous). But if a negative thought is impairing your ability, it is counterproductive to keep thinking it, whether its truth or falsity can ever be determined. For example, lying in bed obsessing over the thought, "What if I'm an insomniac and never again get a good night's sleep," can keep you awake, so it is counterproductive to think it, whether it's true or false. Many pessimistic thoughts are like this one: They are self-fulfilling and therefore not useful thoughts. They aren't true or false. They make themselves true by thinking them.