add_filter( 'display_post_states', 'custom_post_state' );
function custom_post_state( $states ) {
global $post;
$show_custom_state = null !== get_post_meta( $post->ID, '_some_costom_field' );
if ( $show_custom_state ) {
$states[] = __( 'Custom State' );
}
return $states;
}