Include a column in the concept dictionary named "Set Members", which contains the concept IDs of all concepts that are members of that set.
If the field is left blank, this signifies that the concept is not a set.
If the field contains one or more concepts IDs, this signifies that the concept is in fact a set.
(In the same way that the existing "Answers" column can be blank or contain one or more concept IDs.)
This will also allow for concepts to belong to multiple sets.