When you wear a sports bra and you officially don't have any boobs any more.
When there's a gaping gap where your boobs should be in a bra.
When you buy a dress that's supposed to be all cleavage but it's just all sternum when you wear it.
When you still fit into all your pre-puberty tops.
When you try to wear a strapless dress and spend all night hiking it up because your boobs offer it no support.
When you have to buy an entirely different size bikini top to bikini bottoms.
When you've had a particularly good meal and your stomach sticks out further than your boobs.
When you lie on your back and your boobs completely disappear.
When you appreciate the fact that you can lie on your front to sleep.