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.
I've you log in to the CMS it's in the page title or you can go to admin -> plugin manager it shows the CMS version and shows all other modules. If it's 6.1.x.x it's R2 and 6.0.x.x it's 6 standard.
I find the easiest way is to log in to admin and the version number is in the page title so you can see it by hovering over the tab in the browser.
Jon Jones has written about this: http://www.jondjones.com/learn-episerver-cms/episerver-developers-guide/episerver-cms-tutorials/the-quick-and-easy-way-to-find-your-episerver-websites-cms-version-number
Worth noting that if you are running Episerver CMS 6 / CMS 6 R2 then Microsoft will end support for Windows Server 2008 in just over 12 months (that CMS 6 / CMS 6 R2 runs on). So you should consider upgrading to a newer version of Episerver soon.