My Lumia 950 is enrolled in the Windows Insider programme. I'm pretty sure I selected the Release Preview ring, but how can I check to make sure? The Insider app only lets me select a new ring, it doesn't show me the current one.
Open Windows Insider App on your phone. Select a 'three dot button icon' on bottom right of screen below "Learn more about becoming a Windows Insider". Select about option. You will know on which build/ring your phone is set.