Immediately after two separated charged particles are released, their acceleration increases. This tells us that the sign of their charges is opposite.
Work Step by Step
If they are the same, they will repel and both speed up, but the force on each will become weaker as they separate, and so will the acceleration. Only if the charges are opposite, will they attract each other and both speed up. As they get closer, the attraction increases and so does the acceleration of each.