Easy. I can see things, hear things, feel things.
Okay, but what if it's all a computer simulation? What if you were captured by evil scientists that put your brain in a vat. This vat is connected to a supercomputer programmed to trick your brain into perceiving these things? How could you possibly know if anything was real?
That is ridiculous. I don't believe you for one second. I would know if I were in a simulation.
I just would. You can sense that sort of thing. If something were off I would just know.