There are ways to tell if a kid is the actual murder child.

Wanda for example 

Her giant fangs will shrink into smaller more human looking like one

Her hands turn to normal but her nails will be long and sharp as claws and is connected to her bone

She has a small bump on her back where her tail were

In her hair are two bumps to where her ears were

Her eyes are human looking except there a thick circle of black around the iris