The first step in any strategy to finding a specific poem is to gather as much information about the poem as you have available, including the poet's name, the title or sections of the title, line from the poem or parts of line and any unusual words used in the poem. Tailor your strategy for finding the poem according to how much of this information you have available.
With the name of the poet, you can follow links available at poetry or education websites such as the one at About.com. Once linked to the poet's page, use the search feature to look for the poem using the full title, line, fragment or unusual words. If there isn't a search function on the page, search the text on the webpage using the control-F function to search for pertinent text connected to the poem that you are searching for in the poet's page.
If you only know some of these details without the poet's name, these sites that publish poet links may also have listings of periods of poetry, such as classical or modern, in separate archives that can be searched independently. If none of these strategies work, search for a line or the poem's title on a major search site for the full text of the poem.