Plagiarism checker by Small SEO Tools is a good free tool.
One of the best tools is https://www.quetext.com/.
If this one doesn't fit your requirements, try another one: https://www.plagscan.com/plagiarism-check/.
If you want to check the texts for uniqueness, you need to use special programs. They can be found through Google. most of them are free. I recommend checking the same text in two or three programs. so you will be sure that the content is quality
You can use https://www.copyscape.com/. It is a good tool and I hope it helps.
I think you can just use any of the available tools for plagiarism check. For example, I like the one from https://plagiarismcheck.org/ the most, and you might find it interesting to you too. Hope I helped you here, good luck with it ;)
This is the risk you take when you get 3rd parties to write for you.
This kind of work can cause you 2 different issues. One is the legal (copyrights), the second is operational which is penalties imposed for “Duplicated” content by Google or other Search Engines.
The most practical way of finding out duplicated content (which also means copyright issues), is to search for “long-tail sentences” rather than keyword or short key phrase. However, be warned, this tasks requires patience, time, and an enormous amount of diligence. It can take you hours to go through every possible combination, articles, etc., which makes you wonder why you bothered contracting out the work to begin with!
So take a number of key sentences from the work they have done for you, and insert the full text in the Google Search Tool. See what comes up, click through, read the articles, and take action accordingly.
Here is an example. In one of my articles there is this sentence:
“Your Website Is not a Brochure”
You should find the article on the first page of Google, but also many others also appear. You can now check each article and see if there is any copying of the content has gone on. Remember copying concepts or subject matters is not breach of copyright, but copying entire sentences, images, etc. is a breach.
So there is nothing wrong with other people writing about the subject of “Your Website Is not a Brochure”, as long as they do not copy the text from our website.
In summary, this is not an easy task, and personally, we stay away from contract writing unless they are people we know well and trust implicitly.