How to find out which Log4j configuration is loaded?

We're using Log4j (via Slf4j) for logging.

Our log4j configuration is rather simple:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
  <Appenders>
    <Console name="Console">
      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    </Console>
  </Appenders>
  <Loggers>
    <Root level="INFO">
      <AppenderRef ref="Console"/>
    </Root>
    <Logger name="net.sf.jsi.rtree.RTree" level="INFO">
      <AppenderRef ref="Console"/>
    </Logger>
  </Loggers>
</Configuration>

Note that root logger is set to INFO.

However, we're getting a huge amount of DEBUG logs from a third-party library (net.sf.jsi). Logs we're absolutely not interested in. My guess is that there's a different Log4j configuration on the classpath somewhere which gets picked up and sets root logger to DEBUG.

My question is, how could I found which configuration is actually loaded and used by log4j?

Update:

I have seen this "possible duplicate". However, the answer which suggests to use -Dlog4j.debug does not work for me.
I've added -Dlog4j.debug to the VM arguments. I only get:

DEBUG StatusLogger org.slf4j.helpers.Log4jLoggerFactory is not on classpath. Good!
DEBUG StatusLogger Using ShutdownCallbackRegistry class org.apache.logging.log4j.core.util.DefaultShutdownCallbackRegistry
WARN StatusLogger Multiple logging implementations found: 
Factory: org.apache.logging.log4j.core.impl.Log4jContextFactory, Weighting: 10
Factory: org.apache.logging.slf4j.SLF4JLoggerContextFactory, Weighting: 15
Using factory: org.apache.logging.slf4j.SLF4JLoggerContextFactory

which is not enough info for me.