I did a lot of customer interviews. I make sure I don't waste my energy showing my product to people who will never buy.
You should know whether a person is your customer in the first 300 seconds of your customer interview. I am not saying you could, I am saying you should. I have seen too many people spent hours in loops when doing customer interviews. They tried really hard to sell their product to people who don't need their product.
In your meeting with prospects, you will probably spend 3 mins greeting. Then you need to pivot to the real deal. The first thing you need to find out is: what is your problem with X? You don't normally need to ask explicitly, but you need to find out. I always let them tell me a bit about their project then ask them how are they doing their X right now. Most of the time, they will tell you what is troubling them as they answer this question. If not, dig in a bit by asking it more explicitly.
If a person cannot tell you what they are having problems with their current solution, she probably doesn't have the problem that you are solving. In that case, don't force to sell. What a lot of founders do in this situation is not giving up on selling. This always cost you morale big. You can give it a shot since you already know the deal is not there, but back out if your attempt has failed. Your morale is more important than a person who doesn't have the problem that you are solving.
Today I met this a founder who signed up for early access on pubb.at support. He is on his way pivoting, and he doesn't know what he wants he just wanted to " chat and see if pubb.at will help him." I knew he wasn't my customer the moment he gave me that. Startups don't need customers who think your platform might be helpful, startups need customers who know that they have the problem you are solving.
Although I end up spent an hour with the person and took his interview as helping him validate his idea. I gave him the best suggestion, which is to do more interviews and learn more about the startup building process. I didn't force any sale.
Founders, find out whether or not the person you are talking to recognize they have the problem that you are solving. You and your idea will worth millions! Don't waste your time and morale on the wrong person.