Look at what the map is trying to explain. See the "master" concept and what it means or could mean. Without the master concept, the causal chains that make up the map will seem to make no sense. If the map is trying to explain the concept of "science," this is the master concept, and all sub-concepts will reflect some key component of "science." The master concept should always be the first step, and it should be kept in mind at all other levels of analysis.
Look at each concept and the set of causal chains the concept is causing. In each step of the map, the concept is causing something. "Science," leads to chemistry, and chemistry leads to organic chemistry. In each case, the concept caused can be found in the concept in front of it: organic chemistry is found in chemistry as a concept, and chemistry is a part of science. Hence, in this simple case, the causal chain is valid.
Analyze levels of generality. Concept maps are most useful when they are explaining one relatively broad "master concept." Therefore, the motion is from the most general to the most specific. In our "science" example, "science" is the broadest term, it contains the most conceptual content. What the concept map explaining this will do is get more specialized as the map progresses. Therefore, the concepts at the top of the map will be general, those near the bottom will be specific and specialized.
Keep in mind the differing definitions of "validity." In the cases above, validity is defined as proper logical progression. "Nutmeg" would not be a valid form of "science" in our example above for the simple reason that the universal idea "science" does not contain nutmeg. Therefore, the concept map would not be valid. However, another approach to the idea of validity is that of usefulness. While not ignoring factual and conceptual accuracy, a concept map may be technically correct but not useful. The "science" concept map would be accurate if it focused only on psychoanalysis, but such a map would have little use in an introduction to chemistry course for high schoolers. A valid concept map is not one that is only technically correct, but also that which is useful and appropriate for the classroom.