I found that this Skype for Business Online PowerShell oneliner gave me the overview I needed to see if there were configuration issues or wrong settings.
If the migration also includes moving Enterprise Voice workloads and switching to a Direct Routing or Calling Plan setup, you may find it difficult to get a full overview of what state the user is in and if all settings are correct.
When migrating from Skype for Business Server to Microsoft Teams you may find that users are not migrated with the correct features as intended.