Hm. Metaphors... well, if you don't want it to be totally obvious, you could use the character's shadow; depending on which way they're leaning, you could See a monster with a human shadow, or a human with a monstrous shadow.
Generally, I'd expect a scion to be an accords non-entity; technically human, so unless / until they sign on with some Power, the accords doesn't cover them.
In my game, we currently have two scion PCs. One of them chose supernatural pretty much since game session one - and, amusingly, still hasn't noticed. (The creature type in this case has a relatively cheap set of must-have powers - including Human Form - and the PC managed to start the game with all of those powers and refresh to spare.) The other is fighting a long, slow, battle to try to stay human against a slowly-accumulating pile of sponsor debt and interesting circumstances - I'm really not sure where he's going to end up, but the player's certainly having fun with it.