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. Killing 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.'