There isn't a good way to know for sure that something is going to work, but there is alot of research you can do that will give you good information.

1. Cold email/linkedin message then speak to 100 potential customers. Attempt presales. You don't actually have to get presales, but that process should give you a good indication for willingness to pay / desired functionality, how people are currently working on the problem, and what language they use to describe their pain (quoting that back to customers is good marketing copy)

2. Build a landing page, run facebook / google ads, find out how much it costs to acquire an email address. Your real CAC will be higher of course, but this is a good starting point.

I'm a founder of https://www.saturncloud.io/. I started with #2, and found it cost 70 cents in advertising revenue to get a click, ~7$ for an email address, and ~$100-$200 to get someone to signup. When we started we were attempting a low touch sales model.

Since then we've started focusing on enterprise sales, because the willingness to pay is much higher. We did #1 when moving towards enterprise sales, but should have done that in the beginning.