Telescopes keep getting better. Shortly we will be able to detect oxygen molecules of various types (02 is normal, but 04 is also possible)
Oxygen is by nature a very reactive element. It can't exist naturally in an atmosphere unless life is generating it. If there is any photosynthesizing life on a planet, it will create enough oxygen that we can detect it.
We should have something capable of doing this within 25 years, and it won't take more than another 25 years of looking to