is there an equivalent of nwadmin -o findworkflows?
This new post by Aaron Labiosa should help you get pretty close to what you are asking for, here is the link: Locate Nintex Forms in SharePoint via PowerShell
I don't think there is a nwadmin action, but I'll double check.
I think you can use powershell and iterate through each list and determine if the content types have a defined url to the disp form.
Retrieving data ...