It would be under the "Applies to" field in the Catalog Discount, this is a Macro rule editor, and here you can either try to use existing rules / macros, or create your own to do a check.
In your specific case, you can probably just do:
SKU.GetValue("Closeouts", false) == true
Note this assumes the value is on the SKU, not the Page Type, if the booleon is on the Page Type you may need to do something like:
Documents.WithAllData.ClassNames("custom.myclass").Where("NodeSKUID = "+SKU.SKUID).FirstItem.GetValue("Closeouts", false) == true