It would be obvious (see the following link for examples).
That it isn't obvious, that in fact the universe looks exactly like the universe would look if no God created it or anything in it, is a major reason why we don't believe in God. There is only one way to explain this massive contradiction: by claiming God deliberately created everything to look exactly like he created nothing. Which entails he is the ultimate Father of Lies. In other words, if God created this universe, then God is Satan.
But this tactic is ridiculous in any other sphere, so it's just as ridiculous for theists to resort to it (even apart from the fact that it entails they are worshiping Satan). "Gremlins cause all plane crashes but always make it look like things other than gremlins caused them" is not accepted as a plausible argument for gremlins. To the contrary, we accept the more obvious conclusion: gremlins don't cause any plane crashes, those other things do. Ditto, God and "creation."