- a quick google search would return a few key articles that show how to query data from SPO via PowerShell.

https://www.google.com/search?q=sharepoint+online+workflows+powershell&rlz=1C1GCEB_enUS850US850&oq=sharepoint+online+workflows+powershell&aqs=chrome..69i57j69i60.10903j0j7&sourceid=chrome&ie=UTF-8

Nintex workflows still run in the context of the SharePoint Workflow Engine so querying them will return both. You can also get access to Nintex Hawkeye to view your workflows as well, and that doesn't require PowerShell. 

Here is a post to consider -https://gallery.technet.microsoft.com/office/SharePoint-Online-Get-all-40d76baa

Thanks @eharris04 .  Shortly after posting this I used SharePoint Online Client Browser and found that the Nintex WFs are listed under Workflow Definitions [2013], so I will try querying that.