In 1999, John K. Kelly suggested a clever experiment to test if genetic variation in a quantitative trait can be explained just by deleterious mutations maintained at low frequency by mutation-selection balance. Otherwise, some sort of balancing selection would have to be invoked. The question is rellevant to the genetic variation in life span associated with the X chromosome in Drosophila melanogaster. There is evidence that recessive deleterious mutations in the X chromosome make male flies die earlier than females, because only the latter are protected by their second X chromosome. But the question remains whether those mutations are just the expected outcome of a mutation-selection balance or if they are maintained by a balancing selection process, such as sexual antagonism. The application of Kelly's experiment to test the "unguarded X" hypothesis is not straightforward. I resort to computer simulations to determine how it could be applied to help resolve the controversy.