I would actually think evolution a particularly poor choice. If you want to pick one question to ask (and if we leave aside the obvious criterion of easy detectability from space) then you would want to pick one strongly connected in the dependency graph. Heavier than air flight, digital computers, nuclear energy, the expansion of the universe, the genetic code, are all good candidates. You can't discover those without discovering a lot of other things first. But Aristotle could in principle have figured out evolution. The prior probability of doing so at that early stage may be small, but I'll still bet evolution has a much larger variance in its discovery time than a lot of other things.

I wonder if there's any way to estimate how hard it is for an intelligent species to think of evolution. It's a very abstract theory, and I think it's plausible that intelligent species could be significantly better or worse than we are at abstract thought. I have no idea where the middle of the bell curve (if it's a bell curve at all) would be.