What license do you have? If enterprise, you should use Nintex Hawkeye to achieve it (Nintex Hawkeye ). If Standard, then there is no such functionality in Nintex for Office 365.
in case Hawkeye is no option for youHenning Eiben created some blog posts about how to find out which workflows exist in your tenant by using PowerShell. Have a look at his post and see if you can adapt his solution for your requirement: