I test the potency of male plants by making edibles. I have a low tolerance for a grower and mmj patient, but I think this might be effective for others as well. I've made whole male plants into edibles infusing them into coconut milk, or heavy cream. You can make the infusion into any sauce, soup, or desert you like. The other method I have used is to make juice with the flowers and leaves by blending them thoroughly with water and then straining it with a tea strainer. Depending on the potency, the juiced males can sometimes give me an overwhelming but short lived edible high. Other times, I only get medicinal effects and muscle relaxation. This is only an indicator, but it can be informative of the terpenes, medical attributes, and potency level.
Primarily, I select males based on structural traits and hardyness. I never use males with obvious pest or mold/mildew susceptibility. Generally I look for good branching, numerous bud sites, and good flower structure. Of course, the traits I'm looking for vary greatly depending on the project, and whether I'm breeding for indoor or outdoor growing.