Far-reaching technological changes are shaping our society and the ways in which we work. A key 21st-century skill for taking advantage of those changes may be computational thinking (CT). CT aims at enabling humans to carry out more effective problem solving by utilizing concepts of computing and computer technology. For a successful integration of CT into curricula, however, it is important to take assessment into account. We review two instruments that capture CT: the Computational Thinkig Test (CTt), a performance test, and the Computational Thinking Scales (CTS), that relies on self-assessment. We have adapted both instruments from English to German. Using a sample of 202 upper-secondary students from Switzerland, we provide further evidence on the validity of both instruments. To this end, we apply item response theory and confirmatory factor analysis. Furthermore, we evaluate the relationship between CTt and CTS. Both instruments show good properties and may be suitable for assessing CT in German-speaking countries at the secondary level.