How do you find out who is behind a black magic spell?

First, are you sure there is a spell?

Not all streaks of bad luck, illnesses, misfortunes or lost boy/girl friends are the result of magic.

- let me digress here for a moment, magic has no polarity of itself, magic is just power, however if it is raised and directed to the detriment or harm of another or others, it is referred to by those outside the craft as black magic - Sometimes things just go wrong.

If you are experiencing a string of unpleasantness, and you have been dabbling in the craft yourself, this may just be the backlash of the Rule of Three (That which you send out WILL return to you Threefold).

Eventually it will stop, once your three times has run its course.

If you are not a practitioner of the craft and do not know anyone who is, then I find it unlikely you are under any spell at all. Look to yourself.

If you are sure you are under a spell, someone must have planted the idea, check with them first.

Ask what they know about it, and what makes them think it is a spell.

If you are intending to "send the spell back" or retaliate I suggest you re-read the Rule of Three, sending it back is unnecessary as the gods/fates/universe have that well in hand.

As to the retaliation... just don't go there.

Think long and hard on your motivations for searching out this "person", two wrongs don't make a right, learn to protect yourself from further such incidences and trust to the gods for the rest.