How to Tell if Space Is Expanding Between the Two of You

How do you know if space is expanding between the two of you or is it just moving further and further away from you, faster and faster? Maybe they're repulsed by us and they're trying to get away from us. And there's another way to think about it which is to put the whole universe in a single reference frame. But that second view of like thinking about everything in terms of our frame doesn't really work because we can't extend our frame to the entire universe.