The easiest way is coping a phrase or a paragraph and pasting into Google. If you see it was duplicated content there, then your content is duplicated. As you said, you wrote it from scratch thus It is rare to have duplicated.
Other than that, you can use tools as copyscape or other tools out there.
Out there, tons of plagiarism tool that maybe you can found. But of course, not give you the maximum results. If you seriously with your writer to keep from copied, the premium checker its best choice.