How do you tell how big a womens breast size is just by looking?

you really can't just know how big they are unless you see that they are small then you can tell it's an a and if they are pretty big they are probably a d, e ,or c or yo can just ask the person but in a nice way unless if your a guy then they'll just think your a perv... Why would someone ask that kind of a question? But anyways, you can't tell the exact size but you can def. tell if it is an A, B, C, D, etc.