You do the dirty work of contacting people in that industry, asking questions about their problems, and listening (without pitching your product) That might in

You do the dirty work of contacting people in that industry, asking questions about their problems, and listening (without pitching your product) That might involve a lot of cold emails, Linkedin Inmails, Skype calls, coffee meetups, stalking in conferences, joining relevant meetups, listening in on online forums, tweeting people who tweeted about that problem, etc. Then you might a friendly group of 3-4 people who might give you friendly feedback as you develop the product. Keep talking with them. Keep iterating. Thats really all there is to it. There are no “hacks” and no shortcuts. Just work.