How to determine what an app does and if it contains malware?
Short answer: you can't know

A bit longer answer: Simply running and using the application will not show you what might be going on behind the scenes. To dig into that, you'll need to run the application in a debugging environment to analye data and process flows, keep track of any subprocesses created, monitor any network communication initiated by the application etc. etc. As this is rather cumbersome and requires a lot of skills and experience, it usually comes down to trust at the end.