You have an Azure subscription that contains 100 virtual machines. You regularly create and delete virtual machines. You need to identify unattached disks that ...

From Home -> Cost Management + Billing -> Cost Management, scroll down on the options and select View Recommendations:

Azure Cost Management / Advisor -
From here you will see the recommendations for your subscription, if you have orphaned disks, they will be listed.
Reference:
https://codeserendipity.com/2020/07/08/microsoft-azure-find-unattached-disks-that-can-be-deleted-and-other-recommendations/