Display message when there are no Custom fields in post

Try it with get_post_custom_keys().
http://codex.wordpress.org/Function_Reference/get_post_custom_keys

// inside the loop
$custom_field_keys = get_post_custom_keys( $post->ID );
if ( empty( $custom_field_keys ) ) {
	echo 'no custom fields found';
}

If a plugins or theme has also added custom fields to posts you’ll need to check for your custom fields only.

$keys_found = false;
$post_custom_fields = get_post_custom_keys( $post->ID );

// add your custom field keys here
$keys_to_check = array( 'custom_field_key_1', 'custom_field_key_2', 'etc...' );

foreach ( $post_custom_fields as $key ) {
	if ( in_array( $key , $keys_to_check ) ) {
		$keys_found = true;
		break;
	}
}

if ( $keys_found ) {
	echo 'custom fields found';
} else {
	echo 'no custom fields found';
}