A scientific theory is a set of principles that explain and predict phenomena. All theories are only provisional, in that they are upholded every time an observation supports the predictions they make. In principle, a good observation that is inconsistent with a theory's prediction can falsify it.
In this case, Einstein's theory predicts the lunar orbit to be slightly different than Newton's gravitation model. If someone could find a discrepancy in new observation he would have found a flaw in Einstein's theory of general relativity.
As of this writing, no flaws have been found in the theory of general relativity. Though such claims pop up every now and then.