Look for similes first, one of the most common types of metaphor. If a comparison contains the words "like" or "as," this is a simile. For example, in his poem "Lucy Gray" William Wordsworth uses the following simile to compare the movement of a young girl's feet to the movement of smoke:
Her feet disperse the powdery snow,
That rises up like smoke.
Next search out other examples of comparisons between unlike things that do not use like or as. Earlier in the same poem, Wordsworth compares Lucy with a deer:
Not blither is the mountain roe.
Finally, separate what the literary critic I.A. Richards called the "tenor" and the "vehicle" of the metaphor. The tenor is the idea the poet is conveying, and the vehicle is the image she uses to convey it. In "Sonnet 35," for instance, where Shakespeare compares his male companion's faults to a rose's thorns, the faults are the tenor and the thorns the vehicle.