How to check if your site has duplicate content
You can use tools in the SEO industry to check for this. But these generally tend to charge a monthly fee. This can be a huge, time-consuming job without the tool access, depending on your website size and how well the site structure is set up, especially if you’re new to SEO.
You may also get a penalty from Google notifying you that you have a set number of days to fix the issue. You’ll need Google Search Console to access that.