Most Helpful Girl
You can't just predict bra size by just looking at someone's boobs, it's MUCH more complicated than that. The size of the band is very important and that's where it gets confusing.
If anything, look at her bras and look what size they are. If you are buying her bras then good luck because different stores have different sizes.
Most Helpful Guy
Understand how bras fit, and look at lots of boobs, clothes and unclothed, online and in real life. I'm pretty good at it.