Short answer, you can’t. E.g. on the international space station, you could have 1 person “standing” on 1 wall, while another is “standing” on the other. They would both see the other as upside down. Both would be right.

Long answer, we agree on common frames of reference. We already do this on earth. It’s just that in space, we don’t have gravity, and everything is moving around a lot already.

E.g. in a ship under thrust, thruswards is up. We could also use the plane of the solar system, or the direction of the local planet as a reference. All of them are arbitrary however.

Amusingly, if you continue down that rabbit hole, you encounter relativity. It says there are no prime frames of reference, EVERYTHING is relative.