When starting a pilot for Hybrid Azure AD join, it can be useful to keep track of the number of devices that currently are already Hybrid Azure AD joined.
The most straight-forward way to do so is within the GUI of the Azure AD portal:

However, when used for reporting or other reasons, a scripted solution often is a better fit. The below PowerShell snippet returns all devices that are:
- Known in Azure AD
- Joined in a local domain
- Running Windows 10
A requirement to run this script is being connected to Azure AD by using Connect-AzureAD for instance.
Get-AzureADDevice -All $true |Where-Object {($_.DeviceTrustType -eq "ServerAD") -and ($_.DeviceOSType -eq "Windows") -and ($_.DeviceOSVersion -like "10*")}