How do you find a poem when you only know one line?

If you have an Athens account through a literature course in a British University then there are several "line search" publishing services which you can use. MUSE can also be used in the same way. The former are only available to literature students and publishing industry employees, the latter is accessible to students and those who pay for access (it is a journal archive). You can type the line into google or Poemhunter.com and explore the links to track it down. You can also use free expert services.