Hello analogline :)

You can't check through the sim (using a path for example) if a planet has been terraformed. 

In addition, I think that the special planets (Kyros, Hekim, Auriga...) cannot be terraformed.

But if you really want to check "terraforming-situation" of a planet:

My advise is to create a "ghost" quest (you can find several occurences of ghost quests in XML - Example: SophonsQuest-Chapter0):

This quest will not be displayed to the player and will be used in order to check if a specific planet has been terraformed. 

If this planet has been terraformed, then the ghost quest launches the "real" one. 

 <Decorator_PlanetTerraformed>                          

           <Input_PlanetGUID VarName="$PlanetGUID"/>                              

 </Decorator_PlanetTerraformed>

<Action_ChooseOutcome Name="RealQuest"/>

Another solution, as you said, is to focus on buildings. But I think it will be much more complex :/ 

There is a lot of buildings made to terraform a planet. You'll need to know which is the current type of the planet and the next one.