Question

why do we say that only repulsion is a sure test of charge on a body?

Answer 1

Because when we bring a charged body near an uncharged body an opposite charge is induced on the neutral body. And their will an attraction between them. But the given body is uncharged.
Therefore, repulsion is a sure test of charge.

Answer 2

Repulsion is being considered as the sure test of charge on body as the induction of charge is possible between an uncharged body and charged body when brought close together.

Explanation:

The charged body conducts charge to an uncharged body through the transfer of electrons according to the charge on the charged body. Due to induction of opposite charge on the uncharged body, both will be in attraction and thus, the uncharged body will remain in question of the possession of charge.