How to Know When to Follow Up With a Question

How do you know when to follow up with a question? And I always say follow up on something that's subjective, right? So if somebody says something like, oh, the experience was so frustrating. What does frustration mean to that person? Because frustration can mean a whole load of different things. Maybe they're not actually trying to express frustration or frustration is it the right word, but sad is or overwhelmed or something else.