Unfortunately that's not always true. Quality yellow labs are impossible to sex based on looks alone. I was actually over at a friends a while back admiring his 'subdominant male' yellow lab (an F1 from my own WC stock), only to return a few hours later to see his 'male' with a mouthful of eggs. She was even acting like a male, chasing others around and everything. The only sure-fire way to sex yellow labs is by venting.