You can check which service account is currently registered to the FINAL FANTASY XIV Companion App by checking "Service Account Status" in the Mog Station (
https://www.mogstation.com).
If a service account is currently registered, the following will appear under Service Account Status:
Otherwise, if a service account is not currently registered, the following will appear under Service Account Status:
* If there are multiple Service Accounts under the Square Enix Account, (+1 favored destination) will be granted to the Service Account that is currently registered. "Companion app in use" will show for Service Accounts that are not currently registered.
For more information on the FINAL FANTASY XIV Companion App and its features, please visit
https://na.finalfantasyxiv.com/companion/