You can interact with the mitmproxy terminal window to see what is going on. You can use your eyeballs to look for URIs that begin with "https://" if you perfer. But grep seems less prone to mistake.
Any time there's an https URI there, that means that the app has successfully sent or received data over an HTTPS connection with an invalid SSL chain. That either happened because it just accepted the invalid certificate, or because it warned you and you proceeded.