In most cases, you can't directly know if someone screenshots your IG story. Instagram doesn't have a built-in notification feature for this.
Generally, there's no official way for you to be notified if someone takes a screenshot of your IG story. However, some third-party apps claim to offer this functionality, but they may not be reliable or could violate Instagram's terms of use.
You typically won't be informed if someone screenshots your IG story. Instagram keeps this aspect private. So, it's kind of a mystery whether someone has captured your story or not.