If you are an Office 365 Company Administrator, you will receive an email notification once your tenant has been scheduled for a service upgrade. This email is typically sent several weeks before the upgrade is scheduled to take place.
In case you have missed this email notification, you can utilize the Windows Azure Active Directory Module for Windows PowerShell to detect if your tenant has been scheduled for upgrade, and also detect if your tenant has already been upgraded.
Note: This method does not show the exact date of the scheduled service upgrade.
Value in Output
If the tenant has been upgraded, this attribute will contain the date of the upgrade
The current version of the Office 365 service is shown (i.e. - 15)
If the tenant has been scheduled for upgrade, this attribute will equal 'True'
Note: If you don’t see any of the properties shown in the table above, that means you haven’t been scheduled yet.
This did not work for me. The command "(Get-MsolCompanyInformation).CompanyTags" returned "azure.microsoft.com/azure=active". Then I tried just "Get-MsolCompanyInformation" but it returned just company address, phone number, and DirSync info.
You will see the Azure property if your O365 tenant also has an Azure subscription. You won't see any of the service upgrade properties if you haven’t been scheduled (I've updated the post with this clarification, thanks).
Get-MsolCompanyInformation by itself will not show CompanyTags because it is a hidden member.