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."