I think oxygen producing plants probably beat us there.

Ain't no pussy plants makes as much gas as me.

I said more. As in "more than the dinosaurs". I didn't say "the most."

...Whenever I think about people trying stuff like this, with theories on alien life, one thing springs to mind.

That dude who drew some stuff based on heavy biology and it ended up that every large animal in the planet he came up with had what looked like a dick but it was in fact their mouths. And the heads of all of them were where the heart was, otherwise vestigial.

I wish I remembered the link for it, though, but it was stupid.

I know. Just sayin.

I mean, you wanna talk about makin things go extinct? From what I recall, when oxygen started showing up in significant quantities, it was like armageddon.

Yeah, actually we've all been meaning to talk to you about that.

What if there's very little life?

In those cases it would depend on a lot of factors, such as how far away the planet was, how strong the signal was, how good your detector is and what frequencies you are looking at.
A few bacteria on the surface of a plent won't be detectable by this means- you would need multicellular organisms. To be feasible you would need a planet covered in plant-life on a similar scale to earth- that is the calculated example and in this case the effect is strong enough that pretty much any planet we can detect emissions from we could check. Animals would make it stronger.
If a planet was say mostly desert with scattered plants it would be difficult and we'd need to be very lucky.