What can be done to ensure that a black magic specialist is authentic and credible?

“Black” magic is said to consist of spells, special curses, and “the evil eye” that bring harm to one’s enemies. “White” magic, on the other hand, is said by its practicers to produce good results by breaking the spells and canceling the curses.

Here is the real question, why would a person need this type of practice to live a happy life.
In the beginning of man's creation, it was God who created every good thing for humans that we needed to rely on. However one who was against that good, decided to create the evil and have those who work under him ( black magic specialist).

Their authenticity is from, "Their father the Devil" as stated in the Bible at John 8:44
Some people prefer to put their trust in this negative being, rather then put their trust in the Creator who's name is Jehovah (Psalms 83:18)

To anwser your question, No, black magic specialist are not credible!
But the truths found in God's word the Bible is. Please examine jw.org