Log onto your favorite search engine. While Google offers exceptional services, others like, Yahoo, will suffice -- since they all function with similar technologies.
Generate a search phrase by entering your single line of poetry into the search box, then surrounding it with quotation marks. Including the phrase within quotes will ensure that the search engine seeks specific results that include all of the words simultaneously, which will drastically limit what it finds.
Press enter, then scour the results for poems that include the phrase you used for your search. Try to remember the context of the poem to help you identify the correct listing. Can you recall the poet's name? That's obviously a great identifier. If not, scan the results page for verses that seem to ring a bell. Chances are you'll find exactly what you wanted.