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

If charges have same sign force is repulsive

If charges have different signs force is attractive

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.