First clue: they're hideously expensive.
Second clue: They're 18 years old.
Third clue: They have a ridiculous club name.

This last one might take some explaining. You know how on your Manager tab, there's that list of "Clubs Seeking Manager"? Here's the current list it's offering me:
FC Nappy
FC Tidy
Exclusive FC
AC Scabby (Yes, really. "AC Scabby". Scabby!)
FC Gleaming
Tidy FC

So if you see a ridiculously expensive 18-year-old being offered by AC Scabby, he was generated by Nordeus, he's a "Nordgen," and you should break the bank to get him.

Hope that helps.