How do you know something isn't real? How can you tell that that cat isn't really there or that voice calling you doesn't exist?

How do you know that something is real? Do you know that that conversation you had really happened? That car passing by actually existed?

Is our universe and perceptions of reality based on shared experiences alone? How can you tell that something did or didn't happened unless someone is there to confirm or deny?

And how do you know that they are real?