You can’t, however know how or what you exist as, ie in what form you exist.
You could be a brain in a vat in a matrix, you could be a computer program in a simulation, you could be in hell.
The act of questioning your existence means you exist in some form, even if only as a program designed specifically to question it’s existence, but beyond that, you can’t KNOW what is real, what context is hidden from view, or what is deception.