With the preview of the new Office 365 administration portal, Microsoft is delivering new tools too.
The first one is related to the directory synchronization.
The first thing to know is to take advantage of this tool you should run the latest version of the directory synchronization tool: Azure AD Connect (or AAD Connect).
Then, from the server hosting the AAD Connect instance, connect to your preview administration portal and scroll down to the What’s new section to hit the link related to DirSync
Then, if any issue has been detected, you will get a Troubleshoot link displayed near the section which requires attention
This will redirect you to a new page where you can select a quick scan or a full scan. As explained for both options one is quicker and superficial (quick) than the other (full) which also analyze your On Premises Active Directory.
For the purpose of this post, I choose the full scan
This will download a tool which will have in charge of gathering all the information from both online and on premises
Once completed, you have the result with actions to resolve your issue(s)