A thought experiment:
Suppose we are moving in a spacecraft (A), carrying an excess of electrons on spaceship surface, but we can't go outside to measure it, and it's moving at constant speed $v$, with respect to other distant ship (B).
When (A) pass in front of the ship (B), suddenly the two metal ships start to approach at increasing speeds, it's because of the force from magnetic field generated by the relative speed of moving charges .
We have to curb and redirect the ship, and of course solve the original problem, but we don't know about the excess of electrons on our surface, a scientist on board make hypothesis some minutes before the impact and thinks that the problem is a lack of electrons! (and uses some command to increment them, worsening the crash)
How a moving (and electrically charged) observer at constant speed could explain metals coming to him?
Meanwhile on the (B) ship, technicians thinks their ship surface is not charged at all, and the force is because their metal structure itself, they thinks (A) ship is the charged one, and they don't know what to do because they are neutral, and action should be taken at the (A) ship to neutralize it because attraction would occurs regardless of the sign of electric charge.
As it has not sense to ask which ship is moving at speed v, because it's relative, does it have sense to ask about charge sign? Is there some way to tell which sign is the own charge? It's possible to know inside a system the sign of the outside visible charge?
Help those ships captains... before they crash!. =)