Using Groups allows us to organize our Contacts and an individual contact can be a member of multiple Groups (or none). To easily determine all Groups for a particular contact:
- Open the Mac OS X Contacts app.
- Highlight a particular contact.
- Press the Option/Alt key.
Within the Groups list (left column), any group containing the contact will be highlighted. This includes Smart Groups. On my system (OS X 10.12.6, Contacts 10.0), the group titles turn blue.