There are many ways that can help you get an inventory of PST files that are scattered throughout your organization. After you find your PST files, there are tools you can use to copy them in a central location. Having PSTs in a one place then allows you to import them to mailboxes, where you can then apply the rich set of compliance features in Office 365.
You can use the Microsoft PST Collection tool to search your organization's network for PST files. The tool helps you get an inventory of PST files that are scattered throughout your organization. After you find your PST files, you can use the PST Collection tool to copy them in a central location. Having PSTs in a one place then allows you to migrate them using CloudMigrator.
After you're confident that the PST files that you collected have been successfully migrated to Office 365, you can use the tool to delete them from their original location on your network.
The PST Collection tool is available from Microsoft, here.
The advantage of using the data in the Windows registry is its speed. We can quickly find enumerate user profiles and identify PST files loaded in Outlook. Once we have that information, the file information can be checked using SMB. This will require the Remote Registry service to be enabled and TCP port 139 to be open on client computers.
Using WMI is slow compared to relying on the registry, but will locate files that are not open in Outlook. The Windows Firewall “Windows Firewall: Allow remote administration exception” should be enabled so that WMI can be accessed remotely.
Overall, it depends on the environment you are deploying into. In general, the WMI method will return more files. However the PST capture tool has the benefit of being able to move the files to a centralized location.