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.