Gender of a Fly

How to tell the gender of a Fly ? JUST TOO CUTE.
This is the cleanest e-mail joke I've come across in a long while.

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?"

He responded, "3 were on a beer can, 2 were on the phone.