Tracy Boggiano has some advice on configuring and monitoring distributed Availability Groups:
Monitoring can be tricky with distributed AGs because of the way it shows up in SSMS. The distributed AG created on the primary AG does not have the ability to show you a dashboard to monitor traffic like the typical AG and the secondary replicas do not even show the distributed AG. So, to monitor the distributed AG you are best to head to the DMVs. DMV sys.availability_groups has the is_distributed column that will allow you to only see the distributed AGs. Below is a query adapted from the Microsoft Docs query to add additional information to check on the status of the distributed AGs which you must run on the primary AG.
Click through for more details.