Epic coincidence of two bullets colliding

How can you tell?

They were both fired

If they were both in motion, the fired bullet would not have made a perpendicular impact to the bullet at rest. The twisting grooves are plainly visible on the fired bullet, a result of being pressed through a barrel's rifling. The projectile that was not fired shows a ring of decay at the base and slightly up the sides, indicative of corrosion in a pattern consistent with having remained attached to the cartridge case for a long period of time. The large dent on one side could be from a shovel or perhaps pliers when the bullet was removed from the shell.