Question

explain how repulsion is a sure test to find out the charge of a body

Answer 1

like charges repel and unlike charges attract each other... so from this the charge of the object can be found

Answer 2

Attraction may occur between two oppositely charged bodies or a charged body and an uncharged one. That is why in attraction, one is not sure if both bodies are charged, However in repulsion, there are two bodies that have the same charge. That is why repulsion is the sure test of 
Repulsion is sure to test of charge of a body because of it indicates that the other object is charged and has charge opposite to that of first object.