Remember, that in all collisions momentum is conserved; that is, the momentum before the collision will always be equal to the momentum after the collision in a closed system. The difference between elastic and inelastic collision is regarding the conservation of kinetic energy. In an inelastic collision, the kinetic energy before the collision will not be equal to the kinetic energy after the collision. An example of this can be a collision of two cars head-on. On the other hand, in an elastic collision kinetic energy is conserved-meaning that the K.Ebefore=K.Eafter.
In order to determine whether the collision is elastic or inelastic, you need to calculate the kinetic energy before and after the collision.