I've recently been working on applying retention policies to mailboxes and we've been doing a granular roll-out to the users. I've been using some Powershell commands to track who has the policy applied and who hasn't.
The following command will list all users who have a retention policy applied to their mailbox:
You can export that to a CSV if need be by using:
To identify which users don't have a retention policy applied to their mailbox the following command will assist:
Again to export that to CSV you would use: