How can you tell if the force is attractive or repulsive in electrostatics?

The electrostatic force of attraction or repulsion is can be found as follows,

let us consider 2 bodies in gravitational field of earth. Let the distance between those two bodies is equal to r meters. The charge on the first body is Q1 and charge on second body is Q2.
By columbs force, F   α Q1×Q2
                                   r²
F=k Q1×Q2                   where k-gravitational constant.
        r²

Ask me, if you have any doubt regarding this.

If charges have same sign force is repulsive
If charges have different signs force is attractive

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