Deciding on a theme for a literary text requires you to examine the text closely and think beyond the superficial aspects of the text to its deeper and often hidden meanings. It's important to step away from the text after reading it, and think about the message it's trying to convey. To do this you must develop an overall understanding of the text, beyond its entertainment value.
To determine the theme of the text, you must learn to analyze the different elements of the text and reflect on how they work together to bring out the meaning of the text as a whole. In poetry, it could be the sound of the poem combined with the images the poem creates in your mind that help to convey its meaning. In a short story, the characters and their behaviors or reactions to what happens in the plot might convey the meaning that you identify in the story.
Most writing assignments in college literature classes depend upon your understanding of the concept of themes in literature. Although a writing assignment might not stipulate providing an interpretation of the text in question, your interpretation of that text is necessary for writing an effective paper about it. Thus when you're writing about the text in any capacity, you will need to have identified what you find to be the overall meaning of that text.
Discerning themes in literature lends itself to the way you analyze and comprehend information in all areas of your life. Students who learn to detect the meaning of a text in literature in an accurate and efficient way, showing support for their interpretation with evidence from the text in question, can then go on to detect meanings in any text to which they are exposed. Determining themes in literature encourages the development of fine-tuned critical thinking skills, which are crucial to any discipline or profession.