Something that is not obvious from article or posts in this thread, but seems to be taken for granted: how exactly do you know how much of some target molecule

Something that is not obvious from article or posts in this thread, but seems to be taken for granted: how exactly do you know how much of some target molecule you created? The article for example mentions 1g of remdesivir, but that seems like a complicated chemical so how do they tell it apart from something very similar but different? Some kind of GCM or electron microscope?