Question

how can I come to know that an object is positively or negatively charged ?

Answer 1

In order to tell the sign of an object charge, you need another object with a known positive or negative charge. If you rub a piece of glass with silk, it will have a positive charge . If you rub a piece of amber with fur, it will have a negative charge .

and to know about the nature of charge of other objects u should study triboelectric series.

triboelectric series defines which object gains electrons and which object loses electrons. 
and as a result u will get to know the charge on the object.


hope it helps you