If you're looking for something that still works in 2024, you might try a tool called IG Unfollow. It’s a Chrome extension that helps you detect and unfollow users who don’t follow you back — with control over delays to avoid account issues. https://igunfollow.convertinsight.com/
you can use statigram like others said, but you'll risk getting your Instagram account banned. thats what happened to me when i used it for that.
A quick way is to use a trusted third-party app or tool that shows your non-followers. Just be careful to choose one with good reviews to protect your account