One way to tell is if your interactions with them on the story platform seem limited. For example, if you used to be able to like and comment on their stories but now can't. Another clue could be if you don't receive any notifications related to their stories anymore.
One way to tell could be if they suddenly become less communicative about certain events or if you hear about things from others but they never mentioned it to you. Maybe they also avoid eye contact or change the subject when related topics come up.
You could tell if they act defensive or get nervous when you try to discuss it. Maybe they start making excuses or try to distract you from the topic. Those could all be indications that they're hiding their story.
One way to tell is if you can't interact with their posts or comments related to the story. Also, if you search for their profile and the story doesn't show up for you while it does for others, it could mean you're blocked.
It's possible for someone to discover if you send them their story. For example, if it's sent through a platform with visible notifications or if someone else happens to see it. However, if you take precautions and use private or encrypted methods, the chances of them knowing could be lower.