You know your an INTP when: (True story)
So, my boyfriend is an ENFJ. This means that his behavior, moods, and general thought processes make very little sense to me. So I asked him if he would mind if I kept an observation journal on him. He doesn't mind, so I'm going to get a journal and start keeping track of his mood shifts.
So, you might be an INTP if you keep an observation journal on your partner to better understand them.