We do not know how Jack dies on This Is Us, but Mandy Moore confessed on Larry King Now that the main cast including her do know how he will meet his maker during the first episode of the second season.
Then she explained to the Ora TV host, “We all had to know. If you think about it, all of the core actors sort of had to know…because something that fundamentally earth shattering and life changing that happened to the kids at some point in their lives is something they are going to carry with them as adults.”
Therefore, if you cannot wait until September to find out how Jack dies and you see one of the actors in the streets, then stop and ask them because they know. If they do not tell you, then try treating them to some drinks at a nearby bar to see if they will share their knowledge with you. Although, isn’t more fun to see it for yourself instead of being spoiled? Says someone who likes to be spoiled!