If nothing else, telltale traces of our nuclear industries will be clearly identifiable. There was a natural nuclear reactor in what is now Gabon, and the fission products it left behind (e.g. Rutherfordium) are how we know it was there.
For a while, until it all decays. But the total amount of material involved is minuscule. You would be looking for parts per trillion, if indeed you were bothering to look at all. And what would draw your attention to look?
