The most common mistake I see is people building the product first. They fall in love with an idea, spend weeks on it, then go looking for someone to buy it. By then it is too late to change the thing that matters most: who it is for.
We do it the other way round. Crowd first, product second.
A starving crowd has three things in common. They have a painful problem right now. They already spend money trying to fix it. And you can reach them with ads at a sensible cost.
Read the comments under popular videos, the Reddit threads and the one star reviews of competing products. If people describe the same frustration in their own words again and again, you have found real pain.
If nobody is selling to this crowd, that is usually a warning, not a gap. Competitors running the same ads for months tells you the market buys. Your job is to offer something clearer, faster or more specific.
You should be able to describe the person in a sentence and picture the ad that would stop their scroll. If you cannot, keep looking.

"Nobody builds a restaurant and then asks what is on the menu."
Inside AI Franchise this is the job of the AI research team. The starving crowd finder scores possible crowds on pain, spending, reach and fit. The competitor and swipe research skill maps what rivals sell and which ads they keep running. The customer avatar builder writes buyer profiles in the crowd's own words.
You still make the final call. But you make it with a research doc in front of you, not a hunch.