The rise of social media platforms and the subsequent lack of traditional gatekeeping mechanisms contribute to the multiplied spread of scientific misinformation. Particularly in these new media spaces, there is a rising need for science education in fostering a science media literacy that enables students to evaluate the credibility of scientific information. A key determinant of a successful credibility evaluation is the effectiveness of the criteria students apply in this process. However, research suggests that existing credibility criteria are often not integrated into students' actual social media evaluation behavior. This hints to a lack of transferability of the existing criteria. As a consequence, knowledge about how learners evaluate credibility in social media is a first step in closing this gap. In the present study, we report results from six focus groups with 21 10th-grade students (M = 15 years, 57% female, 38% male, 5% nonbinary) about their usage of different credibility criteria in the case of social media posts about climate change. The data were analyzed through qualitative content analysis and as a first step assigned to established credibility dimensions of content (what?) and source-related criteria (who?). Additionally, given the complexity of social media, we also added a composition-based category (how?). In a second analysis step, we adapted our subcategories to the recently proposed credibility heuristic by Osborne and Pimentel. The findings suggest that students generally take criteria from all three heuristic credibility dimensions into account and combine different criteria when evaluating the credibility of scientific information in social media. Based on the application of the credibility criteria to the heuristic, implications for the development of teaching materials for fostering science media literacy are discussed.