I guess I just think it's Ni because:
Introverted Intuition function allows a person to have a sense about the future. It is the ability to grasp and get a sense of a pattern or plan. Information that is usually hard to understand and dissect is easily processed through Introverted Intuition.
I think it's reading the body language and what they are saying and making subconscious connections. It's information that is hard to understand and dissect I think, the connection between body language, what they've said, and past experiences. That allows us to sense the future of the relationship. Sense what type it will most likely be.
Though of course things happen and we are not always right. Just usually right.
Like an underlying potential that you can easily sense in the person. Though it's up to you and that person to get there. And even the smallest changes can have a large effect.