This Site lists all Workshop Content with their ID, for rFactor 2.
Click the Button to load the List.
Hint: to load the List will last around 15 Seconds. 38 Steam Sites will be searched for the needed Data.
HINT: Unfortunately i can not get the needed Data from the Steam Websites that contain the DLC Items.
But @Ricardo Diz did add a Switch to display the ID on his Site, that uses the Steam API, so you can get it there.