We elaborate on a new approach of assessing content-based epistemic clarity of college physics students in terms of their ability to discriminate between different epistemic warrants for propositions in a chained argument in physics. A threefold classification (nominal, physical, and mathematical) of warrants is used, with each class split into a number of distinct epistemic categories. The assessment tool consists of a questionnaire, illustrated here for a standard undergraduate physics derivation, in which the student chooses the appropriate epistemic category (from an appended list) for each proposition in the given derivation. The tool includes a brief supplementary orientation note for clarifying the meaning of different categories. It is tested for reliability and internal consistency. Correlation of content knowledge, ascertained independently, with epistemic clarity is investigated. Surveys on large and diverse samples of physics undergraduates carried out with or without the orientation note reveal a variety of epistemic errors, some common among all groups of students, and some that are good discriminators. The epistemic measure obtained from such a tool is, however, limited in scope by its narrow definition. The construct validity of the tool and the pedagogic relevance of the work are discussed.