Suppose that both Earth and Mars was booming with life in a similar way. Except that, on Earth, there was no hominid family, and no humans evolved. Instead, humans evolved on Mars.
Assuming we had similar levels of technology, civilization, and science at similar times (e.g. agriculture ~10K years ago, telescopes ~400 years ago, physical turing machines ~80 years ago, etc), and that life on Earth was almost exactly the same, except without hominids, how soon would we have discovered life on Earth?
Also, more broadly, how quickly would we have figured out that something strange is going on with Earth that makes it different from the other planets? (Without necessarily knowing that it's life.)