I think the main thing is that you have to actually talk to the people you want to sell to to find out what their problem is and why it's still a problem despite their next best alternative. Then you need to talk to them about the potential solution you have in mind to make sure you're not off the reservation entirely.
But the real moment is getting a specific person to actually give you money, hopefully before you put in too much work on the experiment. Promises or "that sounds cool" won't do it, you have to prove it to yourself by getting a person to give you real money for your thing, that's the only way to really know.
This is a really important aspect. B2B apps start out as semi consultant firms relatively often as it’s somewhat easier to simply promise you’ll solve a problem for a company for a certain amount of money than convince them that a new app will solve their problems.
But the real moment is getting a specific person to actually give you money, hopefully before you put in too much work on the experiment. Promises or "that sounds cool" won't do it, you have to prove it to yourself by getting a person to give you real money for your thing, that's the only way to really know.