How Can You Tell the Sex of Flies

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. Killing any?" She! asked. "Yep, 3 males, 2 Females," he replied. Intrigued, she asked. "How can you tell?" He responded, "3 were on a beer can, 2 were on the phone."