We cannot prove that something does not exist or at least we can push certain things into the 'concept' or 'fiction' domain. Anything outside of those must exist. To you, a person on the other side of the lmao pipes - I may be a concept or some AI. You can test me. With well chosen tests, you can conclude that I am either a man-made construct or an actual being like yourself. For this to work however, you have to believe that YOU exist.