Ask HN: How do you talk to potential customers?

So I have an idea that is very niche focused. I am already building a version 0 (70% complete). Within the next 2-3 weeks, I hope to have it ready to be able to demo.

This app will have 2 types of users. One that will consume the niche content for free while the other will be recruiters who are interested in finding those niche individuals. The idea is to build a solid user base which will attract these niche recruiters.

I work in the same niche and have a lot of good personal contacts already. The question is: How do I start talking to these potential customers? I mean the medium. Sure I can call them up and send email to start the conversation. But I wanted to ask them more specific questions around their pain points etc. What do you guys use for this ? Survey monkey with questions ? What else ?