As I read now again, linking verb refers to the speciality of the subject, so it seems it is 'IS'(to be) verb in the sentence, but at first, I thought indicate refers to it(because of the test). I guess I was wrong.
a genius is a name. At first, I coudn't see it and just saw the genious.
A genius may be predicate nominative so there is not any predicate adjective here.