Professors are very familiar with their subject areas and what has been published in relevant journals, books and on web sites. In addition they are also aware of what has been written by other students in their classes over time.
With this experience the following situations may lead a professor to suspect plagiarism:
Purposeful plagiarism is extremely risky, and unintentional plagiarism carries the same consequences. Learn about researching and citing properly, and don't be afraid to ask Mount Library staff for help!