You...can't? At least, we can't. Sometimes Gideon could tell us if something was fixed. Gideon was our ship's AI. But I think it depends on how much the death of a person's death impacts the world. My father and I were supposed to die at the same time as my sister. When I was plucked from the timeline and ended up our ship, it changed my fate, but that didn't have much of an impact other than to save my father. My sister still died, and I can't change that, because her death leads to so many other things happening. I Captain a timeship, but I can never bring back my sister.