If indeed it is a bikini or tankini that is based on bra size and you aren't willing to settle on one that doesn't base itself on bra size then the easiest option is to ask her mother (awkward but explain why and that its a surprise gift) or her friends, most girls will discuss what bra size they are with each other when discussing clothes, if the friend doesn't know think of a way she could find out subtly in conversation about clothes.
It is nigh on impossible to find out a bra size just looking because there is 2 things to take into account, circumference of the back under the breast and around the rip cage and then the cup size.
Which is why it is known like 36 D, it means 36 inch circumference around the rib cage under the breast and a D sized cup (at least that is the method here, I know size calculations vary wherever you go).
Some places go by 3 measurements. link
You have to keep in mind even girls need to be measured in shop often to take into account there breasts changing shape and there weight altering and being young and still developing she may be a different bra size in 6 months time, not so easy to just look and guess I'm afraid.
Again if all else fails just buy a bikini that doesn't rely on bra size.