How can I determine what log configuration source Logback actually used?

If you want to go deep into Logback, you can do the following

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.core.joran.util.ConfigurationWatchListUtil;

public class Main {

    private static final Logger logger = LoggerFactory.getLogger(Main.class);

    public static void main(String[] args) throws Exception {
        LoggerContext loggerContext = ((ch.qos.logback.classic.Logger)logger).getLoggerContext();
        URL mainURL = ConfigurationWatchListUtil.getMainWatchURL(loggerContext);
        System.out.println(mainURL);
        // or even
        logger.info("Logback used '{}' as the configuration file.", mainURL);
    }
}

It will print the URL of the loaded configuration file.

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.