So how can I find out?Look in Server Templates under "Apache Website". If you have the options labeled "Redirect webmail.domain to Usermin?" and "Redirect admin.domain to Virtualmin?" then you have the feature. I don't think it is limited to Professional. It was mentioned in the changelog when it was introduced in the most recent version (or maybe 3.59).
and also there must be other people contributing extensions to the GPL version. How are they organized?Not so many, so far. Most of the extensions have been written by Jamie. But all are available for Virtualmin GPL, too. Nothing outside of Virtualmin core is specific to Virtualmin Professional. Even the mobile theme has been available for GPL for a while now. They are searchable on the Webmin modules page, and are also available in our GPL yum repository and wbm repository.
http://software.virtualmin.com/gpl/ (look in wbm or universal for our modules)
If you installed Virtualmin GPL using our automated install script, all of the same plugins that are installed for Professional are automatically installed, and the yum or apt-get repository is setup for you for easy access to updates and the additional modules that are not installed by default.
We certainly encourage folks to write additional modules and plugins. Jamie's written quite a lot of documentation on the subject in the developer section of the documentation wiki.