How did you determine at least one is a girl? Presumably you looked in some way. But did you look at one child or both? That's the crux of the ambiguity.
I think you are asking "how did the person who told you there is at least one girl learn that".
The answer is: it doesn't matter how because that is an unambiguous statement.
It means "you can assume the family does not have two boys".
I think people are actually getting hung up on "you are told" as if that could be a lie, or some kind of trick, when it is really just supposed to mean "here is some more information that you can rely on".
It's not a question of wealth but of efficiency. Even if you took every single meter of high speed rail that France has built in the last 50 years, you would barely cover 2/3 of the distance of a single line between New York and LA.
https://en.wikipedia.org/wiki/Brightline