That quickly boils down to “How do we know anything?” and the answer to that is “We don’t”. When you think hard enough about anything you can come up with an explanation why what we think to onow and believe is wrong. To get around that irl you can employ different tactics. For example, you can check how plausible sonething is. How many assumptions do you have to make for a theory? Usually, more assumptions means less plausible. And you can ask yourself " why does it matter? What would it change for me?" and the answer is most likely it doesn’t and nothing.