Well, one way to tell is if you get notifications that someone interacted with your story. Another way could be if you have analytics available and it shows activity related to views. But keep in mind, not all platforms provide the same level of detail about who exactly viewed your story.
You might notice if there are likes, comments, or shares on your story. That can be a sign that someone viewed it.
Sometimes, the platform you post the story on will have view counts displayed. If it shows a number greater than zero, it means someone has viewed it. Also, if people mention or talk about your story to you, that's a clue they saw it.