As described on this page:
If you subscribe to the presence of a person who is not on your contact list (also called temporary presence subscription), the subscription expires after 68 minutes. After the subscription expires, you must re-subscribe to the person’s presence in order to continue to receive presence updates.
Fetch method for any known-existing jabberwerx.Entity object. If the object does not already exist, `undefined` is returned.
That should tell you if the contact is in your EntitySet. If you don't get 'undefined' as a result, that means that contact is in your roster. The "node" is optional, and you don't need to provide it.