What Questions Could I Ask Someone Who Doesn’t Seem to Have True, Saving Faith?

Questions about what questions to ask someone who claims to be a Christian but doesn’t seem to have true, saving faith and when to stop trying to convince someone who denies some essentials of the faith that he needs to be saved. 

• What questions could I ask someone who seemingly has a said faith rather than a true, saving faith?
• At what point do I stop trying to convince my grandfather, who denies some essentials of the faith, that he needs to be saved?