You can't walk into a shop and guess at your partner's size. A woman's bra is almost like a second skin, so if you choose the wrong fit she'll never wear it. If you don't know her measurements and would like to keep your lingerie shopping a surprise, all it takes to is a little Bond-style espionage to find out. Simply take a look at the laundry when the lucky lady isn't around and make a note of her labels.
With bras, there are two sizes to keep track of. The first is the cup size, which can range from A to K. The second is a number, which relates to the measurement around the body and rib cage. It's important to note that the two numbers are linked: for example, a 32D will not have the same size cup as a 38D because the letter relates to the volume of the bust itself and the size of the cup in relation to the band. You won't be able to buy a 32D bra for a woman who wears 38D, even if you get it altered. Bra sizes can vary across lingerie brands, so it's a good idea to check a couple of bras that your partner wears most often – these are most likely to be the ones that fit her best.
If you're really struggling or don't feel comfortable peeking into her wardrobe, you could try and find a negligee or camisole instead. You don't need to be as exact with the sizing for these flowing garments.