What I would like to know is how good is our ability to actually detect life from afar? For instance, we only discovered water on local planets and moons within

What I would like to know is how good is our ability to actually detect life from afar? For instance, we only discovered water on local planets and moons within the last decade or two. How loud do aliens need to be for us to notice them? If we were near Alpha Centauri, would we be able to detect life on Earth? Would a SETI scan of Earth detect our radio waves or other signatures of life?