Before start to use this tip, I thought that the easiest way to see what modules your Apache is using is to grep for the LoadModule in httpd.conf
These days I found a smart way to print out the Apache modules that are running on the server. Here is the command to run from shell (for Gentoo):
apache2 -t -D DUMP_MODULES
Tip was tested in on Gentoo, CentOS and FreeBSD machines and works fine.