Meh, it's a long way from saying that "we don't know how the O2 could have been produced except by life" to "the O2 was produced by life." Just look at the current debate about methane on Mars.
In a 1993 article, Carl Sagan considered the atmospheric oxygen content "at least suggestive of biology" (Conclusions, 3rd paragraph).
http://www-pw.physics.uiowa.edu/~dag/publications/1993_ASear...
Am I supposed to disagree with that? High atmospheric oxygen content is suggestive of biology. So is seasonal blooms of methane on a planet like Mars. Is that suggestive that there is life on Mars? Yes! Can we definitively infer that there is presently life on Mars? Not yet.
So life on Earth is pretty visible from the other planets in our solar system.
And spectroscopy to observe the composition of atmospheres of planets orbiting the nearest stars will probably happen in not-too-distant future.