How can I tell what group(s) a contact is already in?
There is no such field in the card, but you can create Smart Group to include contacts that are (or are not) members of certain groups.

To see all contacts that are not in any group, just select Card is not member of any group.

These options are available only if you have more than one group in your contacts.



Alternatively, you can do the following:


Open the Mac OS X Address Book.
Highlight the desired contact, either under All or in a specific group.
Press Alt / Option on keyboard.


Any groups of which the contact is a member will be highlighted in the Group column.