How hard could it be?
I very much love an old idea of dumping plutonium, tons of it every year in our beloved Sun. Nothing bad will happen because of that, only the light of our dear life giving star would be marked this way by some absorption lines.
So, every advanced alien who noticed this strange occurrence would be forced to conclude, that something odd is going on. Plutonium cannot arise in a star by some natural process. It must come from a planet. How and why?
The most sober conclusion for an alien would be – someone intelligent is signaling he is there!
Actually, astronomers have searched for “unnatural” absorption lines in many stars’ light around us, long ago. Nothing has been found. Aliens don’t dump plutonium on their stars.