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 replied.

"Oh!  Killing any?" She asked.

"Yep, 3 males, 2 Females,"  he replied.
Intrigued,  she asked, "How  can you tell them apart?"
He  replied, "3  were on a beer can, 2  were on the  phone".