kinematics - How to tell if the collision is elastic or inelastic?

I'm a programmer and a game developer, not a mathematician or a physicist. So please go easy on the math :)

I know two things:

However what I need to know is: given the mass, the velocity, and the 'angle' the two objects are going two be when they collide - how can I know if I need to compute an elastic or an inelastic collision?

I need this for the 2D game I'm developing, that tries to simulate relatively-realistic physics. Thanks for your help