The online FAQ mentions a system property "log4j2.debug", which should print all internal logging to the console, if set to any value.
Here's the relevant quote:
From log4j-2.9 onward, log4j2 will print all internal logging to the console if system property log4j2.debug is defined (with any or no value).
But in reality, the internal logging can only be enabled by setting "log4j2.debug" to some case-variant of "TRUE". Please update either the documentation or the code.