Athlynne

You could try using or adapting methods once used to detect changelings - Faeries left behind in the place of stolen human children. Here are a few I remember off-hand - filling the child's room with smoke; tying a hair from a black cat with a red ribbon and placing it around the child's neck; placing the child on a shovel and holding it over a fire.

You can find a lot more of these, I'm sure, if they sound helpful.