That's not a simple question. In fact you can get a Ph.D. in just that - epistemology - the science of certainty. With evidence, in particular, you have to rule out every other thing that could possibly have caused the evidence to exist, and if you can, then, and only then, can you infer what you do from it. And the inference has to be logically sound. Renee Descartes first opened up that can of worms for us. He asked the same question you did here, of himself, and realized he couldn't be certain of anything - then it dawned on him that at lease he must exist because there he was thinking about it. Hence his famous quote - I think, therefore I am. But even with that, he allowed that he could be dreaming and/or we could all be in a Matrix - Brains in a Vat, as he put it.