Recently, I got a question from a customer to list all users that already enrolled in Azure MFA (through, for example, the url https://aka.ms/mfasetup.
The following PowerShell code lists all non-disabled users that already enrolled in Azure MFA:
Connect-MsolService
Get-MsolUser -All | Where-Object {$_.StrongAuthenticationMethods -ne $null -and $_.BlockCredential -eq $False} | Select-Object UserPrincipalName