Actually, the presence of gaseous oxygen in the atmosphere of an Earthlike planet is often cited as a strong sign of active life.

The reason is that oxygen is highly reactive, and won't last long if it's not constantly being replaced. On our home planet, the only significant source of gaseous oxygen is photosynthesis.

All else being equal, if we ever detected a planet with a strong free-oxygen signal in its atmosphere, we'd have strong evidence for life.