How can I measure the volume of my audience and the extent of their pain? (I will not promote)

Hi! I’m validating my ideas (I will not promote) Idea is make particular infocourse for designers. I see that people are interested in this, but how to understand that this idea is actually will be relevant on high volume. Problem for me now - nobody/or small amount of people doing that. Or this blue ocean, or this idea difficult to sell as it to narrow How you validate your ideas? And next I’ve got an idea to reach out people who’s doing the same or similar info courses, how you would reach them? thx so much!