Answer:
Determine if the hair length is heritable,
Explanation:
If to check whether the long hair are adaptive in the males or not, one should check whether the trait of long hair is also found in the offspring or not.
If this trait is found in the offspring, also check whether it provide some level of fitness to them or not.
Thus, if this trait is heritable which means that it passes to the offspring, then the trait has adaptive value.
Hence, by checking whether the hair length is heritable or not, one can test the given hypothesis.