As important as knowing something is knowing what you know, such that you can compare what you know to what there is to know, and identify areas to focus on.
Setting out what there is to know (i.e. a complete syllabus) is relatively easy, and should be standardised per subject area to enable individuals to map what they know to the what there is to know in a standardised (and hence comparable) fashion.
Syllabuses are typically hierarchical datasets, with subtopics sitting within topics, and subsubtopics sitting in subtopics, etc. However, cross-linking between topics, particularly for dependencies (i.e. prerequisities) is essential.