Is there a command that list all enabled Apache modules?
You need to enable the info module:
sudo a2enmod info.load sudo a2enmod info.conf sudo service apache2 restart
will provide a long list of modules, and configuration info.
To view from remote servers, you can change the 'Requires' option in /etc/apache2/mods-available/info.conf to allow remote servers to view info.
The above answers are old and no longer work for my modern Fedora Server 31 / 32 and Apache 2.4.
Here's what does:
httpd -t -D DUMP_MODULES
But, there is a caveat that this will only work if you have an appropriately configured /etc/httpd/conf/httpd.conf, so if you're in the middle of editing to set LogLevel, it won't work if your edits are in-progress and not valid!