Here’s a quick tip to display the names of associated backend SQL primary and mirrored (if any) database servers of a Standard or Enterprise Edition Lync/Skype for Business Server 2015 Front-End pool:
Get-CsUserDatabaseState -RegistrarPool fepool.leedesmond.com
The result lists the Identity, Mirror and Online status of the SQL database servers of the specified FE registrar pool. This works since only this pool type houses the user store (database) which keeps key user related information like presence and routing data.