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