Searching for evidence of critical thinking in discourse has roots in a definition of critical thinking put forth by Kuhn (1991),  which places more emphasis on the social nature of discussion and knowledge construction. There is limited research on the role of social experience in critical thinking development, but there is some evidence to suggest it is an important factor. For example, research has shown that 3- to 4-year-old children can discern, to some extent, the differential creditability  and expertise  of individuals. Further evidence for the impact of social experience on the development of critical thinking skills comes from work that found that 6- to 7-year-olds from China have similar levels of skepticism to 10- and 11-year-olds in the United States.  If the development of critical thinking skills was solely due to maturation, it is unlikely we would see such dramatic differences across cultures.
I liked Tom’s reference to the rest of the iceberg of critical thinking. Many people are confused by creative and critical thinking or see them as being exclusive. I was very pleased to come across a blog entry that understands their interdependence. I think one of the hardest components of critical thinking is the analysis. The part where you are trying to understand what is going on WITHOUT drawing any judgments. If you can do this well, your judgments about what is relevant or accurate or important or whatever will be much more complete.