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!'