You're right that CO2 is not very reactive. But the fact Earth has a lot of O2 says there's some process replenishing it as it's very reactive. The amounts of CO2 changing in proportion to the amount of O2 would point to something processing the CO2 into O2.
As long as our theoretical aliens are aware of photosynthesis, which can use a number of pigment chemicals, they would likely settle on photosynthesis. The longer they watch Earth they'll see seasonal variations in CO2/O2 levels.
As long as our theoretical aliens are aware of photosynthesis, which can use a number of pigment chemicals, they would likely settle on photosynthesis. The longer they watch Earth they'll see seasonal variations in CO2/O2 levels.