The latter way is the most correct one. Even if you wrote your paper independently, you should know how to check a paper for plagiarism. Everyone can unintentionally plagiarize (and this really happens very often), so you need to be sure that your paper is unique, non-plagiarized.
Thus, let us consider the most common ways of how to check a paper for plagiarism.
How to check a paper for plagiarism: free online services
Most of online services that check papers for plagiarism are free. So, this is the simplest way to check a paper for plagiarism before submitting it. Plagiarism detectors will highlight all phrases in your text that match those found online. So, you can easily get rid or paraphrase all those parts to avoid accusations of plagiarism.
How to check a paper for plagiarism: paid services
Naturally, there are also specialized plagiarism detection services for which you have to pay. They check your paper not only against the online database, but also against their own database of books and journals. Such services are mostly used by colleges.
How to check a paper for plagiarism: search engines
Never underestimate search engines, especially powerful ones. It is enough to put a couple of sentences or a paragraph from your paper into the search field, and engine will report on the same phrase on the web.
Thereby, we recommend you to write papers on your own and check them for plagiarism to avoid any misunderstandings and accusations of what you actually did not do.