Is there an easy way to pull a report of ALL Marketo forms being used?
Unfortunately, there isn't, since "used" can mean "embedded in a 3rd-party site" and those forms may not have had any submissions during the past 90 days.
If you want to see all the forms that have been submitted, including embeds, you can use the Bulk Extract API. It's a simple query as these things go, but you will have to be/have a developer.
Retrieving data ...