Not one but I'd check if the host where the program is used from is in a Windows domain, check for non-conventional local IP ranges (10.x.x.x or 172.x.x.x), or use some other information like the e-mail used on the record, is it free @gmail or @bigcompany? You can 'wargame' this, if you want.
At the end of the day, you should probably still 'suggest' they buy licenses. More aggressively each time as more and more signs come up. Maybe mail yourself about who could be a company and reach them over the phone to ask?
