You'll see them in the JSON response under properties - if there is no value in the contact for a given property then it won't return that property in the JSON i.e. if no first name is there for a contact firstname won't be returned.
You'll see them in the JSON response under properties - if there is no value in the contact for a given property then it won't return that property in the JSON i.e. if no first name is there for a contact firstname won't be returned.