One of the tasks of reviewers is to verify that code modifications are well tested. However, current tools offer little support in understanding precisely how changes to the code relate to changes to the tests. In particular, it is hard to see whether (modified) test code covers the changed code. To mitigate this problem, we developed Operias, a tool that provides a combined visualization of fine-grained source code differences and coverage impact. Operias works both as a stand-alone tool on specific project versions and as a service hooked to GitHub. In the latter case, it provides automated reports for each new pull request, which reviewers can use to assess the code contribution. Operias works for any Java project that works with maven and its standard Cobertura coverage plugin. We present how Operias could be used to identify test-related problems in real-world pull requests. Operias is open source and available on GitHub with a demo video: https://github.com/SERG-Delft/operias