If it is connected to air terminals or any objects with conenctors in then it wil pick up that it is part of a default air system, whether it be Supply, Return or Exhaust.
A simple way to tell with is which is to place Duct connector families at the start of your main duct runs run, ensuring that you have the duct connector families set correctly with Supply In, Return Out, Exhaust Out for example. Then by adding filters to each view based on system type, you can colour code each different duct type. Just make sure you close off the ductwork.
You don't have to make specific systems out of these ducts as the connectors will automatically make them default systems.
If you don't want to do systems then you could add a project paramter to ducts and duct fittings to specify the duct type and combine that again with filters to colour code. However I would advise against this method as it is a time wasting exercise due to the parameter being reset to nothing almost everytime you modify the duct layout. Hence you have to keep repeating the task of adding the information into the parameter.
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