How Can We Know if We're Looking at Habitable exo-Earths or Hellish exo-Venuses?

The differences between Earth and Venus are obvious to us. One is radiant with life and adorned with glittering seas, and the other is a scorching, glowering hellhole, its volcanic surface shrouded by thick clouds and visible only with radar. But the difference wasn’t always clear. In fact, we used to call Venus Earth’s sister …