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,

Spoiler:

"3 were on a beer can,
2 were on the phone."