Way to tell someone has a Cygnus?

To answer the topic, there's no way to tell by looking at a character whether or not it's a parent to a cygnus. The only way you could tell is if he opened his skill book to beginner skills and the Blessing of Sprite skill was visible.

For that matter, you can't tell a Cygnus apart from a normal class just by looking either, unless they have their Cygnus medal on.