Elementalborn can come into existence in two ways.
Either the bloodline carries blood of a genie, though that is very rare.
The “more common” (while still rather rare) way of them coming into existence is a ritual.
When an infant is born with an inborn illness or birth defect, there is a specific ritual that can be performed by a child's parents (whether by blood or spirit) as long as the child is no older than two years old, this ritual transfers the disease to the parent performing the ritual, usually resulting in their death.
The child is transformed through this ritual to take on traits of an element relating to the humor that was the cause of the disease.
Ifrit for the fire of yellow bile
Oread for the earth of black bile
Sylph for the water of phlegm
Undine for the water of blood
These two types are easy to tell apart. The “created” kind usually has eyes corresponding to their mortal ancestry, although with an iris covered in a colour corresponding to their element.
“Real” elementalborn usually have eyes that are spheres of a solid colour corresponding to their elements. Often glowing.