Read a line of text aloud a few times until you can say it conversationally, as if the words were yours and you were speaking them to a friend. For example, read lines 6-8 of “I, Being Born a Woman and Distressed,” a sonnet by Edna St. Vincent Millay: “So subtly is the fume of life designed, / To clarify the pulse and cloud the mind, / And leave me once again undone, possessed.”
Underline each syllable that you speak a little longer and louder than the others. For example, line 7, when read conversationally, would sound like this: “To CLARiFY the PULSE and CLOUD the MIND.” The louder and longer syllables, in capital letters here, are the stressed syllables. The quieter, quicker, unstressed syllables are the ones you have not underlined, like the "i" in “clarify” or “and.” Most of the poem is iambic, which means its meter uses an unstressed syllable followed by a stressed syllable, a pattern that reads like “da DUM da DUM da DUM.” If you were not sure whether to stress the last syllable in “clarify,” you could guess based on the iambic words around it that the last syllable is probably stressed since the middle syllable is unstressed and the next word, “the,” is also unstressed.
Use an online dictionary with audio pronunciation to check the stress in words you are not familiar with. For example, line 3 of this poem uses the word “propinquity,” meaning “nearness.” Play a recording of this word on a free online dictionary website like Wordnik.com to hear where the louder and longer stressed syllables are. In this example, the unstressed syllables are the first and third syllables: “proPINquiTY.”