How Do You Know When the Answer Is No?

How can we know that the answer to a prayer is no? In my book, i feel like we have to draw a distinction between god's responding to a prayer and god's answering a prayer. God answers a prayer just in case god actually brings about the thing that was requested. How do you know when the answer is no? Saint paul was confident, but he had some kind of a connection to god. He felt like he was the recipient of some direct revelation from god. And most of us don't feel that way. We don't know. So just because the thing doesn't happen, does that mean the answer no? I think it might be just as hard to know