How to know if one has this ability.
Well, uh… you will hear voices that are distinctly not yours. Like another person is speaking to you, not just your own internal monologue in your own voice, which seems to be without any specific physical location in your brain.
For me… usually I classify clairaudience as coming from a specific part of my brain, from somewhere located in a kind of centre-posterior segment of my physical brain. Sometimes it seems like it activates your physical sense of hearing, sometimes it does not.
Personally, I think we all have it. Just like sports, some have it a little more refined than others, but we all get better with practice, regardless of where we start out. I liken it to an underused muscle. It’s there, but takes some use to get stronger.
My opinion is that the best way to get better is to be in a receptive state (like meditation) and call on spirits for other work you’re doing. They can interact with us even if they aren’t mostly or fully evoked. These will come as impressions, pictures, and/or single words and build over time.