How to Tell the Sex of a Fly

A woman walked into the kitchen to find her husband stalking around with a fly swatter "What are you doing?" She asked.. "Hunting Flies" He responded. "Oh.! Kill any?" She asked. "Yep, 3 males, 2 Females," he replied. Intrigued, she asked.. "How can you tell them apart?" He responded. . . "3 were on a beer can, 2 were on the phone."