Question

how do you come to know where is the positive charge and where is the negative charge on a rubbed object OR which object carries which charge??

Answer 1

The best way to determine the charge on an object is the movement of that object in electric field. So, we can not assume that which object will have which charge. It depends on nature of material used.

Ex:

When an ebonite rod is rubbed with fur the rod gets negatively charged because the friction between the fur and the rod makes the ebonite rod to gain electron from the fur. This is because, electrons in fur are less tightly bound than the electrons in the ebonite rod.

So, in this ebonite rod will move towards positive electrode when it is kept in electric circuit.