Maybe it knows from phone usage, e.g. lowered volume while the kid naps.
Zuck for example has a lot of knowledge about his users. He could in theory even see a relationship grow: from initial contact, to increased frequency of messages, and then how the phones are connected to the same WiFi network but without any messages being exchanged (because the people are at a date spot), culminating with one phone spending the night connected to the other's home network..
Hah, imagine using all the gathered data to predict if relationships will work or not..
I'm wondering if this 3rd-party AI will be like smartphones - you need one to participate. Hopefully local ai can help... maybe?