Glad it helped!
If it's an operational layer, typically it might say 'layer X could not be added to the map'.
If it's a basemap or embedded content it'll stop loading the page and redirect. As you've found it can be a bit trickier to identify which layers, apps or content aren't shared.
You can use the browser developer tools to watch the network traffic. I just looked at the requests it made before it redirected to the authentication page. You could do the same thing in an incognito or private browsing session.
