For future reference: if a genuine detective ever comes to talk to you, the first thing he will do is show you his badge and a picture ID. He will do this even if you don't ask him. And he should give you plenty of time to look it over. He will probably also give you a business card. He will not be offended by, or impatient with, your desire to make sure he is who he says he is. Identifying himself is a regular part of his job. And he knows that he is more likely to get useful information from you if you trust him. And he will want you to contact him again later, if you have any new information.