Board Thread:Support Requests - Community Management/@comment-10090661-20130907175018/@comment-4018164-20130907180208

The Installed extensions section in Special:Version shows what you're looking for.

About your second question, i suppose that Wikia checks MediaWiki's website for installation instructions when installing requested extionsions, and if an extension is based on another one it must be mentioned in the extension's page. Though to be sure, you can mention it when requesting extensions installation.