Right now, the only way to tell if a class descriptor, method descriptor or field descriptor is that or another object is to read its properties and see if it kinda looks like a descriptor (e.g., does it have a type property). To make overloading easier and more reliable, we should brand them. We can do this through an own @@toStringTag property that identifies these as "Class Descriptor", "Method Descriptor" and "Field Descriptor". This should resolve tc39/proposal-decorators-previous#24 .