Finding Signs of Life from afar

If I were to attempt the colonization of a planet with preexisting life, how would I manage to pinpoint a candidate planet from the comforts of Earth's orbit? I am interested in the stars somewhere within 40 light years from Earth, and with relatively realistic technology (no FTL drives or time machines).

I am familiar with the technology behind spectrography, but it seems rather unreliable in terms of launching a multi trillion dollar mission. This suggests` that quite a few gasses that are correlated with life on Earth might not indicate life on exoplanets- so, how can we be certain enough of life to send thousands of colonists to a distant star?

Different from this question, in that I'm asking about a much larger scale, and not trying to comprehend a trope.

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Finding Signs of Life from afar

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