You know something real when you see it right? Of course you do, it's real. You can touch it, it physically exists, and you can feel its presence. It cannot possibly be a figment of your imagination, it just can't be.
You catch the end of your finger, you bleed. You know that it is real. If it was a dream you would have woken up. You feel the warm blood trickle down your finger towards your freezing cold palms. You lick the wound, only to be faced with that familiar metallic taste. This can't be a dream, you know it. No dream is this vivid.
Look in the mirror and what do you see? Yourself. As you remember it. But what's that? Something has changed. You brush it off. You must have done something that you don't remember; it's the only reasonable explanation. Wait, what if really, nothing has changed? What if what you are looking at is a mistake, your mind not correctly remembering how you look? Your mind filling in the gaps for something that it is unsure about. But that would mean that this isn't real. But it has to be real, everything is so lucid.
Think, when you close your eyes, and finally drift off to sleep, does that not feel just as real? Only when you realize that you are dreaming, do you finally wake up. But what is it that helps you know that you are dreaming? The sound of your alarm clock, buzzing loudly in your ear? Or a stimulus within the dream itself, something that you look at that brings the realism of your situation into question.
Everyone you look at, everyone you meet here. In this "reality". We are all sleepwalkers. Every last one of us.