We could detect oxygen in the atmosphere, which could indicate life, as oxygen will rapidly disappear from the atmosphere through oxidation without life resupplying it. I don't think we have any built or planned telescopes that can detect oxygen 3000 light years away though. This article[1] discusses detecting oxygen in the atmosphere of exoplanets 5 pc away, about 16 light years, using the James Web space telescope that is yet to be launched.