1. Look from straight front, are the boobs wider than the body? If narrower but got bulge, could be pushed up.
2. Look from the side, draw imaginary line from the front of her legs, to tummy, to chest. How far out is the bulge from your imaginary line?
3. While she's not suspecting, bring her to a very windy place. Even loose cloth will reveal the body shape when strong wind hits.
4. Go for a water activity date. Waterfall/sunway lagoon whatever as long as whole body will get wet. Even if she wears t-shirt, the drenched shirt will give way to the shape.
5. If you have an adult sister, even easier. Find out your sis cup size, then bring her out together, when they sit next to each other, you can compare the size easily and know whether they are larger or smaller than your sis.