As explained earlier, you can detect plagiarism by using a plagiarism checker or you could do it manually. It isn’t unusual for students to read through their papers to find plagiarized parts. That being said, it is very easy to miss plagiarized content when trying to detect plagiarism manually.
This is why it’s best to use a plagiarism detection tool before or after you perform the plagiarism check manually yourself. Similarly, it is worth hiring an experienced academic writer or editor who could check plagiarism for you (both manually and with their own professional software).