I got this question a couple of times over the last months, so now I will create a blog post on how to check it.
What is Office 365 MDM Coexistence: that means that in your tenant your end user can be either in Office 365 MDM or in Intune MDM.
How does it works??
If a user has and Intune license when enrolling the device will end up in Intune, if the user do not have a Intune license the device will end up in Office 365 MDM.
How do you check if your Intune is in coexistence with Office 365 MDM:
There is 2 ways of seeing it – depending on the Intune portal you are using.
First using Microsoft Endpoint Manager admin Center
On the home page click Device Enrollment
You can also use the direct link https://devicemanagement.microsoft.com/#blade/Microsoft_Intune_Enrollment/EnrollmentMenu/overview
Then if your tenant is in coexistence with Office 365 MDM then it will show:
MDM authority: Intune + Office 365
Second using Microsoft Azure portal
Click Device enrollment
Then if your tenant is in coexistence with Office 365 MDM then it will show:
MDM authority: Intune + Office 365
Hope this will help
Read more:
Set the mobile device management authority
Microsoft Intune Co-existence with MDM for Office 365
Mobile-First Cloud-First 
And the second question from customers, will users move between them? If license is assigned or removed for user account.
No, it is only on enrollment.
So if we enrolled in o365mdm the device has to be wiped or factory reset and re enrolled in intune?
Second can we switch from intune mdm auth to o365&intune mdm auth ?
Hello,
In a situation right now where the authority is set to only O365MDM yet Intune is available in the tenant (M365e3 licenses).
This is not my first Intune deployment and normally there would be a banner to click on that allows us to change the MDM authority but that is not present at this time. Its like were stuck with just O365MDM.
Ive contacted support but they are giving the impression that they cant change it right away.
Any thoughts or work arounds?
Did you find a solution for that ? how to change authority ?