How Do You Choose

If I were to travel to Mumbai, Dallas, Jeddah, and Jerusalem I feel sure I could find four folks that were sure their religion was correct and the other three not correct. If you accept the view that at most one of these is correct (a proposition I think most religious folk would accept), how do decide which is correct? You cannot ask these folks, each disagrees with the others. You cannot adjudicate their holy texts; on what basis would you make the decision? You cannot judge on the age of the religion, all are old at this point (and there are older ones). You cannot judge on the number of followers. All have some millions. How do you decide? I would guess that there would be some external measure that would apply. But I do not know what that would be.