How to tell the sex of a Fly

A woman walked into the Kitchen to find her husband stalking round with a fly swatter.

What are you doing she asked ?
Hunting flies he replied.
Oh killing many she asked ?
Yep 3 Males and 2 females he said.

How can you tell them apart she asked ?
3 were on the beer can and 2 were on the phone. He replied.

Tony

Claret Red +4