Ask yourself, "What did I learn from this?" Literary themes can be extracted by determining the moral of the story.
Identify repeated interactions between characters that suggest a theme. These interactions are often verbal. For example, the witch, a stranger to the children, is repeatedly overly kind and giving to them, in spite of her wicked intentions. Her words support the theme that if something looks too good to be true, it likely is.
Look for imagery that is repeated in the text. Imagery can be visual (sight), aural (sound), olfactory (smell), tactile (touch) or gustatory (taste). Imagery can contribute to the overall theme by repetition. For example, the inexplicable presence of sweets and good food that the witch gives the children supports the idea that some things really are too good to be true.
Consider the interplay of a work's overall moral implications, character interactions and imagery to determine a text's theme or themes.