The best time to check your paper for errors is before you turn it in.
PaperRater.com also offers FREE plagiarism detection. Simply upload or copy and paste your paper into the box. Select your education level, 1st grade through Doctorate. Enable Plagiarism detection and select continue.
The free version does have some limitations:
Maximum 5 pages per paper (longer papers can be broken up into separate submissions)
Maximum 50 submissions per month
Maximum 10 plagiarism checks per month
Despite these limitations, it is a great tool for writers of all ages. Try it out for yourself the next time you have a paper to write.