You can do user testing, you can call up people in the industries and ask them, you can do a Kickstarter and spend a little on advertising it to see if it gets any traction, but mostly the reaching out to people in the target market and compensating them for a user interview
I built a very niche monitoring product for sysadmins. Actually I built it for myself three years ago, but I want to explore if others would pay for it.
If you are in the space, or anyone else who reads this I'd be grateful to receive some feedback on a 10 minute video call.