If you need a quick way to identify users in Azure AD that are sync'd from On-Premises AD whose accounts are disabled, you can use the following script to query the "BlockCredential" property for each user in Azure AD ($true means that the user account is disabled).
Foreach ($User in (Get-MsolUser -All))
If ($User.BlockCredential -eq $true)
Write-Host $User.UserPrincipalName "is disabled" -ForegroundColor Red
Read more at source: http://blogs.technet.com/b/fromthefield/archive/2015/06/19/office-365-azure-ad-identifying-disabled-users.aspx