Misinformation and biased opinion-formation plague contemporary politics. Fact-checking, the process of verifying accuracy of political claims, is now an expanding reseach area, but the methodology is underdeveloped. While the journalistic practice of fact-checking is by now well-established as an integral part of political news coverage, academic research requires more stringent methods than what journalists thus far have used. In order to advance the scientific study of fact-checking, we propose two variants of an index measuring the information density of verbal political communication. The main index combines three dimensions: (1) factual accuracy of political claims, (2) their relevance and (3) the magnitude of observed communication. In the article, we argue for the significance of each of these components. Depending on the research problem and data, the indices can be used for comparisons of political actors across different contexts such as countries or time points, or in non-comparative situations. Using examples, we demonstrate that the indices produce intuitive results.