Chlorophyll would be a strong indicator.
I’d be more impressed if they could find life in a human womb.
The problem is how will you keep someone alive long enough to get there. Its not like there’s a Walmart out there to stop and get gas and snacks and oxygen...rebreathing apparatuses won’t do the job for that long of a trip. What if they start out when the planet is primitive but by the time the get there they’ve advanced enough to strip citizens of their rights and become O’bummerland 2. Its not like they can whip that wagon around and head back home. I say we just take care of our own planet, depopulate voluntarily to keep from killing and wasting our own planet and we’ll be fine...: )
There could be other chemicals that duplicate the function of chlorophyll in other environments, which if we detected would not mean “life” to us.
All plaints are not created equally some take more time than others to have life.
The big caveat is mentioned in the Conclusion:
Finally, we show that the “red edge” of chlorophyll absorption at lambda [wavelength] of approximately 0.7 um will be extremely difficult to detect, unless the cloud cover is much lower and/or the vegetation fraction is much higher than on Earth. Assuming extraterrestrial chlorophyll to have the same optical properties as the terrestrial pigments, and assuming Earth-like cloud and vegetation coverings, detecting chlorophyll will require a SNR [Signal-to-Noise Ratio] approximately 6 times higher than for diatomic oxygen, equivalent to a SNR greater than or equal to 100 at R [dimensionless spectral resolution] approximately 20. The detectability only approaches that of O2 if the cloud covering is zero, or if it is light and a much larger surface fraction, 30%, is covered in vegetation.