If two objects repel one another, you know both carry either positive charge or negative charge. How would you determine whether these charges are positive or negative ?

Explanation

To determine the polority of charge on the two objects, we bring one of the objects near a positively charged glass rod. If the object is repelled away from the rod, it must be positibely charged (as like charges repel). However, if the object is attracted towards the glass rod, it must be negatively charged (as unlike charges attract).