You can see the conditions of the other world, and see the composition of that world. This allows you to see what elements are abundant. Assuming there is water on that other planet, you can use the elements that are found there and see what organisms on earth are composed of those elements or use them.
Specifically, extremophiles (types of archaeabacteria) often exhibit characteristics that seem suitable for other planets. Using this, you can estimate what life on other planets have as characteristics, or even what they are made of.
Organisms with similar molecular makeups + organisms in areas with similar environmental pressures often exhibit anatomical + behavioral similarity. You can use this to model this.
