Immediately change the subject. You could say something like 'Hey, that's not a good topic. Have you seen any interesting movies lately?' This shows that you don't want to engage in such inappropriate talk. Also, it's important to convey that this kind of story is not acceptable in a normal conversation without being rude.
Tell them firmly that you are not interested in hearing such vulgar stories. You could add that it makes you uncomfortable and that you would prefer to have a positive and professional conversation. For instance, 'I'm not into this kind of stuff. Let's keep the conversation clean and work - related.'
Well, sometimes it's obvious when they get defensive or change the subject quickly. Also, if their story doesn't add up or has contradictions, that could be a sign.
You can tell if someone hid their story if they have inconsistent accounts, are overly vague, or suddenly become reluctant to discuss the topic. Maybe they also try to change the subject quickly.
One way to figure it out is to notice if their story has inconsistencies or if their body language seems tense or closed off. Also, if they get defensive when you ask for more details, that could be a clue.
You can look for evasive behavior or avoiding direct answers. Also, if their body language seems nervous or they're reluctant to expand on certain aspects, it could indicate they're hiding something.