The following command can be used to determine if there has been configuration drift on a system:
Test-DscConfiguration –CimSession $session
The command only returns true or false. Is there a way to retrieve a list of what those differences are?
Unfortunately, not at this time. You can run Test-DscConfiguration with the -Verbose switch and the verbose messages may tell you what is not in compliance. You should also be able to run Get-DscConfiguration and compare the existing state to the configuration document to see what differs, but there is no nice object structure with the components out of compliance.