“Do we ever know if someone completed his or her mission?”
Seforim say that most of us humans today ore gilgolim (people sent back to the world to make up for a sin we did in a previous life as a different person) but if someone is niftar it should be assumed that the person completed his mission in this temporary physical world & is now ready for the true world above.
i.e. if a child is C”V niftar young there can be many reasons why but one of them is cause s/he might need to make up for causing someone previous pain & is sent back to do teshuva with vice versa of giving people nachas as a young loving child serving Hashem.