2. Check the Audience Trait for the data source. For your cross device data source, you would notice that an Audience Trait has been created that shows the number of ID synced devices for that data source.
This trait's graph looks like this:
3. Contact your Adobe Consultant or Customer care team to set up a bulk export of Customer IDs, their corresponding Device IDs and segment IDs. This way you will have a list of all IDs that you have in your instance of AAM. You can look up into this list and search for your test Customer IDs and the devices it got synced with.