In the Parsha

When someone suspects that he might have צרעת, he must go to a כהן.

If the כהן himself is an expert in this area, he will determine if it is indeed צרעת or not.

Otherwise, the כהן must have an expert examine the person, and determine if it is צרעת.

In either case, if it is found to be צרעת, the כהן declares the person to be a מצורע.