how do you define "using Nintex Workflow" exactly? Does it mean there are workflows running on the site? Or is it already enough if the Nintex feature is activated on the site?
I think both can be checked by a PowerShell script.
I hope you have server access so you can write/execute PowerShell scripts?!
Then it would be possible to:
The WorkflowAssociation object will hopefully provide all information you are looking for. I wonder if you can see if it is a Nintex or a standard SharePoint workflow. Will have to try this myself.