One way to test it would be to see if the median face, by some choice of metric, was also more attractive. The problem with other averaging methods, like simple pixel averaging, is that you can produce a result that is not actually particularly similar to any face anyone has. For example, if you had a population where everyone's face was highly asymmetric, but 50% in one direction and 50% in the other direction, the average would be a symmetric face that is completely atypical for the population.