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.