Consider, that person X due to some reason did bad spell on person Y. How can Y or other person Z can identify that bad spell is happen on Y by X. Specially if X did spell that make Y ill or mentally unbalanced? Is there any way to find?
Please note that this question already assume that black magic is real, you can find various places and resources where existence of black spell is proven.
superstition
, hence you may want to retag with that.