One way to tell is by the feedback you get from readers. If they are engaged, keep coming back for more, and recommend it to others, that's a good sign.
An effective story usually makes the reader feel something - be it excitement, sadness, or inspiration. Also, if it makes them reflect on some aspect of life or changes their perspective even slightly, that's a sign it's working. Another indicator is if the story sticks in their mind long after they've finished reading it.
You can tell if your story is effective by looking at how well it conveys its main message or theme. Also, consider if the characters are memorable and if the plot keeps the reader's interest throughout. If you find yourself constantly thinking about the story and wanting to know what happens next, chances are it's effective for others too.