Question

How do we know if a charged object is positive or negaative through the electroscope method?

Answer 1

An electroscope is a device used to detect the presence of charge and it's relative amount.  One method of charging the electroscope is by conduction or contact.  This means that a charged object must actually touch the electroscope and transfer charge too.  If the electroscope returns to normal after being touched by the charged object then it is quite possible that the charged object is so weakly charged that it is not willing to share its excess charge with the electroscope.

Here are simple clips of an electroscope being charged by conduction.   To avoid confusion, focus on only one at a time.

When charging something by contact it is important to note the following properties

The objects must actually touch and transfer some electrons.The objects become charged alike.The original charged object becomes less charged because it actually lost some charge.  Therefore, there is a limit to how many times it could be used to charge something without being recharged.