How do I know if a contact is a user or an iPhone user in the address book?

I am programming an address book, I want to recognize a Facetime / iPhone user in a contact list. I found that some of the contact details have a "Facetime" clause. I have already checked the framework. but only found a call to the "kABPersonPhoneIPhoneLabel" property, how to judge if a contact is a Facetime user? Thank.