In nuclear fission reactions, a nucleus with a large mass splits into smaller nuclei, typically accompanied by the release of neutrons. Therefore, the correct way to identify a fission reaction in equation form is:
A nucleus with a large mass will be on the left side, and there will be neutrons in the equation.
This reflects the fact that the original large nucleus (such as uranium-235 or plutonium-239) is present on the left side of the equation, and the reaction often produces neutrons as well as smaller daughter nuclei on the right side.