There are many sites which can help you to check content for plagiarism.
A couple of famous and widely used tools are:
There are few more which can be considered as good options:
I would advise you to hire a content writer who is a native English speaker/writer. And then as a safety check use the above URLs to see if the content written by the writer is unique or not. 99% of the time it is seen that the content written by a professional content writer is never copied and is always unique.
Hope this helps you.
Had just the same issue only a year ago :) There are dozens of not bad online checkers, but most of them can't boast with a quick search and 100% precise results. If you're looking for the best online service, here is my favorite https://plagiarismcheck.org/. They at least have a transparent description of how they find plagiarism in texts, so that I am always sure I get the most precise data (anonymously by the way). Another advantage is unlimited text volume, you can insert as big text as you need, no limits for the quantity of symbols.
Grammarly and Quetext. It is better to have a good content writer who can write about unique concepts not already on the internet.
I know that https://plagiarismcheck.org has a good software to check texts on plagiarism. You can use it for SEO and even check your essays. You just install it and that's it.
The most used website for verifying plagiarism on the internet is: Copyscape.com