Forum posting sites are for sharing your knowledge to others related to your expertise. Form platforms should be used for sharing knowledge. For example this forum is SEO knowledge sharing platform, same forums will be there in your niche,
To find relevant forum platform and share and get knowledge related to your business niche should be a purpose to register in forum sites.
Here is the trick to find forums in Google
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.