After selecting an ArcGIS reference layer, then leaving Edit mode, there's no obvious way to find out which reference layer(s) have been selected.
The only way I've found is to go back into Edit mode, get to the ArcGIS Reference Layer search and repeat the search I used to choose a Reference Layer in the first place. Then I can see my chosen Reference Layers are checked. It's hardly intuitive.
Thanks for the feedback. We will work with our design team to see what can be done to help users see the current reference layer, view it's source content on ArcGIS. Thanks for your feedback on what's not working so well for you - it will help us make this a better product!