[fredley]

The similarity index measures things that can be worked out without needing a precise image of the planet. These are things like radius, density and surface temperature, which can be measured by examining its orbit and how much light from the star is blocked as it passes.

[TheBeardKing]

We can also determine the rough composition of its atmosphere by using different sorts of spectroscopy.

https://en.wikipedia.org/wiki/Astronomical_spectroscopy