No, really, how does May know this? Someone upthread says by observation, but observation requires interest, and May is the last person one would expect to be interested in anyone else. She is a snotty little punk, albeit a charming snotty little punk, and that is usually a mark of someone self-centered. Self-centered people are, almost by definition, little interested in others, and therefore usually little observant of others.
Possibilities:
1. May is actually much older than she looks, and has acquired knowledge of humans just by being around a long time. This is unlikely, but I like the image of someone very old inside that little doll body.
2. May is less self-centered than she seems, and Jeph has already laid down hints of this that I have missed or forgotten.
3. Jeph is using May as a mouthpiece to explain what he thinks is so, speaking through her without regard to her character.
4. Jeph will now develop May's character and show how she can be so sharply observant.
5. AI's outsider status requires them to observe humans carefully in order to fit in at all, even when they are personally self-centered.