Recently, we were working on a migration to Microsoft Teams from Skype for Business Server and one of the last steps was to change the Tenant Mode to TeamsOnly.
However, customer asked to check if all domains (200+) had the correct DNS records before executing the change to TeamsOnly.
For this we created a simple script and now it’s part of the UC Lobby Teams PowerShell module.
The best option is to use the PowerShell cmdlet without any parameters:
Test-UcTeamsOnlyModeReadiness
Alternatively, we can use the domain parameter to specify a domain that is part of the domains added and verified to a Microsoft 365 Tenant.
Test-UcTeamsOnlyModeReadiness -Domain uclobby.com
Note: Using the Domain parameter does not require to be connected with Microsoft Teams PowerShell module. Also, we use a different method to check if the domain is enabled for SIP, hence the difference in the output.
UC Lobby 
Good you have published this, very much needed. I needed this just over a year ago when i was having to check 130 domains to move to TeamsOnly mode for a customer. Also the GUI doesnt help, only PS shows what is failing and wasnt that obvious at the time. Was Joao who i spoke to when i raised a Sev A for this, that i understood the logic and very dauting when you have 10s of domains to check.
Sorry for the delay and hope it helps in the next big migration.