You can set a Java system property to output Logback debugging info:
java -Dlogback.statusListenerClass=ch.qos.logback.core.status.OnConsoleStatusListener
This is further explained by the Logback documentation for automatic status printing (very bottom mentions forcing status output) and the logback.statusListenerClass property:
In the absence of status messages, tracking down a rogue logback.xml configuration file can be difficult, especially in production where the application source cannot be easily modified. To help identify the location of a rogue configuration file, you can set a StatusListener via the “logback.statusListenerClass” system property (defined below) to force output of status messages. The “logback.statusListenerClass” system property can also be used to silence output automatically generated in case of errors.