I believe that the aspect of a story that can mostly help you determine its theme is the attitude of the characters.
The way the characters (specially the main ones) are portrayed, with their feelings, thoughts and actions, show us what the author was trying to convey in his or her story and, therefore the intention behind it.
You determine the aspect of the story to determine the theme by the attitude of the characters , or the tone of the author or narrator. So that will help you determine the theme of the story.
*Hope this helps!