Gives you the list of devices connected to Azure AD via Hybrid Azure AD Join. If no results means no devices joined. (see some example results below) -
Additionally you can remotely connect to AzureAD Connect PowerShell from one of your Domain Joined devices. From Azure AD Tenant perspective, there is no configuration that shows up in (AzureAD / MSOnline ) powershell that shows you these details except dirsyncconfigruation of your Domains .