I don't think there is an expression or condition to check whether an instance is global. So you'll have to create your own property. Maybe a family instance variable, that you set to true with various "on created" triggers of your global members of the family.
If you only need to test one UID at a time, you could also create another family but only add your global "Cork" ("CorkGlobal"). You could then have a function that test if "pick by UID" returns 1 instance in both family, indicating that your "Cork" is also a "CorkGlobal".