How do I know if author field was changed on post save?

Best Hook for that would be wp_insert_post_data

function wpse44358_filter_handler( $data , $postarr ){ 
    //do your stuff 
    //if ($data['post_author'] == ...
    return data; // make sure you return something 
}

add_filter( 'wp_insert_post_data' , 'wpse44358_filter_handler' , '99', 2 );

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