Critical thinking means to apply proper statistics to classically heuristical tasks, such as assessing a claim's truthfulness and relevance. In critical thinking, there are two terms you need to memorize: validity and reliability.
Validity means that what is claimed to be measured is actually being measured. This means controlling for every conceivable factor that could exert influence - for every uncontrolled factor, the claim's capacity for being generalized drops exponentially. A claim with multiple uncontrolled factors is extremely unlikely to be universal. While there is no truth, if something doesn't measure what it claims to measure for sure, it may as well not be true because it is not actionable knowledge.
Reliability means that the findings can be reproduced under different circumstances, such as in a new location, conducted by another experimenter or run by another researcher. A study that fails to be reproduced may have been falsified, a freak occurence or otherwise unable to generalize for reasons not necessarily related to validity.
In short, critical thinking means you need to be sure that the claim means what it says it means, and that it wasn't just an extreme. Critical thinking is not a per-subject discipline: in this case, if the man doesn't learn to catch the fish, there will be no fish. Every piece of information you encounter can be thought of in a critical manner, no matter how big or small - if you can chunk it, you can criticize it.
