How do you tell a boy panda from a girl?

This article from the LA Times discusses the cross-strait panda debate between China and Taiwan. And just so you don't have to click another link, here's the relevant quote:

'Pandas are notoriously poor breeders, with a baby panda the ultimate zookeeper prize. On the flight from Beijing to Heathrow in 1974, Brambell recalls spending much of the time shining a flashlight on the animals' private parts to establish that the two were actually male and female. "We wanted to make sure we really had the goods," he said.'

("the goods" hahahahahahaaaaaa!)