I am trying to create a RagePluginHook mod that checks whether a mission has been completed before continuing with the script. Is this possible? If so how?

There might be a better way and I cannot help you with the mission hash, but CHAL_MISSION_GET_NUM_GOALS_COMPLETE might help you. Assuming at least one goal needs to be completed to pass the mission (another assumption on my end), this might be what you need. You could check how other game scripts make use of it.