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?”

He responded, “3 were on a beer can, 2 were on the phone.”