Hi octavioP,
If a repeating field does not exists, get_post_meta($post->ID, 'wpcf-xxxx') will return an empty array: array();
You can try this:
... if (!get_post_meta($post->ID, 'wpcf-xxxx')) { ...exists repeating field ... }
More help: http://codex.wordpress.org/Function_Reference/get_post_meta
get_post_meta($post_id, $key, $single);
If $single is set to false, or left blank, the function returns an array containing all values of the specified key.