By finding fossils out of place -- a fossil rabbit in the pre-Cambrian, for example.
On the contrary. Finding fossils out of place would better serve as evidence for those who adhere to no intelligent design. Adherents of intelligent design would expect to find order in the fossil record from smaller to larger; more waterbound creatures up to those that need air. And so the fossil record testifies.
The fossil record also seems to indicate the Law of Gravity was present and active when it was laid down, just as it is today. Smaller creatures sifted downward with more alacrity than the big critters. I would suspect, if the whole world were immersed in water today the same thing would happen. You would be a fossil on top, and your so called ancestor would be below. Would you predict any differently? If so, why?
Design is a good thing. Especially when it is intelligent and used to good purpose.