How to return newly added terms when using wp_set_object_terms

You can add a filter for a specific function call like so:

add_filter( 'set_object_terms', 'checkNewTerm', 10, 6 );

function checkNewTerm( $object_id, $terms, $tt_ids, $taxonomy, $append, $old_tt_ids ) {

    // Do stuff, ie. compare count of $tt_ids and $old_tt_ids
}

wp_set_object_terms($id, array($migrate_case), 'pa_case');

remove_filter( 'set_object_terms', 'checkNewTerm', 10, 6 );