Guinea pigs are sooo cute, aren't they!?!? I have two:
Cadbury's a year old chocolate and white "semi longhaired" boar, and Bullet is a six month old poorly marked torti and white shorthaired sow. They're both adorable! They had babies a little while ago (by accident actually), and they're adorable too. We only have one of them left with us, though.
blue_heeler - both sows and boars have nipples, just like humans! A boar will have a kind of like a "pimple" "down there", and it looks a bit like an "i", while a sow won't have a "pimple", and it will look more like a "Y" or a "v".