Tricks to find guest posting site on search engine.
keyword “submit a guest post”
keyword “guest post”
keyword “guest post by”
keyword “accepting guest posts”
keyword “guest post guidelines”
keyword "guest post"
keyword "guest blogger"
keyword "writers needed"
keyword "blog for us"
keyword "write for us"
For more details, this guide will help you a lot: https://moz.com/ugc/15-smart-ways-to-find-guest-posting-opportunities
You can use duplicate content checker tool like copyscape or duplichecker or plagiarism checker.
You can do it manually by copying your content in " " and search in Google, you will find those sites in results that have copied your " " content.