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.