Are there any other places except for episerver.config where you can find out which version ("EPiServer 6" / "EPiServer 6 R2") of EPiServer the site was built on?
Usually I look in episerver.config and compare those numbers to the ones in Deployment Center to find out if it's "EPiServer 6" or "EPiServer 6 R2".
I have only been working with EPiServer 6 and EPiServer 6 R2 so those versions are the most relevant to the question.
Edit: Updated the two first paragraphs with what I mean by the word version.
As other have stated, the plugin manager in admin mode, or simply looking at the title of the editor/admin mode browser window shows you the version.
Also, here is a list of version numbers and their corresponding release "Name": http://epiwiki.se/configuration/episerver-version-nummber-vs-release-number
EPiServer version number vs release number (taken from the assembly EPiServer.dll)
The version numbers are built with the following pattern:
Major.release.buildnumber.days
Where:
Major - is the major version that probably has breaking changes to a previous version
Release – the release number should not include breaking changes
Buildnumber – the build number
Days – days after the build number was generated