Get list of all Active Directories in Azure
Both options below will retrieve the available tenants, however the Azure AD Cmdlet will provide a little more information such as the Display Name and Verified Domains of the tenants.
Azure AD Modules
Cmdlet: Get-AzureADTenantDetail -All $true
Documentation: https://docs.microsoft.com/en-us/powershell/module/azuread/get-azureadtenantdetail
Azure Rm Modules
Cmdlet:Get-AzureRmTenant
Documentation: https://docs.microsoft.com/en-us/powershell/module/azurerm.profile/get-azurermtenant
Testing:
If you want to try these Cmdlets without needing to install them locally, check out Azure Cloud Shell. It'll provide you with a browser based Command Line for Microsoft Azure.