Originally Answered: How to know size of bra by lookin at it?
On the tag it will say it.
But by just looking at a girl, you definitely can't tell. Guys will tell you, Oh a flat girl is an A, big girls have C cups, and porn stars have D's or DD's. They're full of it. No number of "years of experience" with women can let you determine a girl's size unless your experience comes from being a bra fitter. Only then can you make an estimate, which may or may not even be close to being right.
Bra sizes are determined by two measurements: The ribcage and the full bust.
The number is the measurement of the ribcage rounded to the next even number
The letter represents how many inches of difference there are between the band size and full bust measurement. A being 1in, B 2in, C 3in, D 4in, DD 5in, E 6in, F 7in, FF 8in, and so on.
Keeping that in mind, bra sizes are all relative. Cup sizes are relative to the band size. Meaning that a girl with a 28DD is not, in fact, very well endowed. The smaller the band size, the smaller the cup size. When you take the cups from a 28DD and but them on a 34 band, they become a 34B. Not big at all.
Most people might estimate a petite girl with an average chest to be about a 34C, when she is more than likely somewhere around a 30DD or 28DDD.
If that's what you meant by this question, you have your answer now- you definitely can NOT tell a specific size just by looking at a girl. Even if you're a trained, professional bra fitter, there is still room for a lot of error unless you're taking measurements and having her try sizes. And if you're just a random person I HIGHLY doubt that's the case. But there you have it. :)