Was SSRS installed in SharePoint Integrated Mode? I've inherited an old QA database server with SQL Server 2008R2 (yes, yes, I know it's beyond EOL; there's nothing I can do about it!) and SSRS installed.

It's the back end of a SP server on a different VM.

How do I tell whether or not SSRS was installed in Native Mode or SP Integrated Mode?
Did you try looking in the Reporting Services Configuration on the server in question? 

https://social.msdn.microsoft.com/Forums/en-US/a856488b-fbcd-479e-9167-34ddf7ba3e83/how-do-i-find-out-if-ssrs-is-intregrated-or-native?forum=sqlreportingservices