I am not aware of a single formula or procedure that can be applied to identify the theme in all short stories, largely because there is so much variation in short stories. The differences in structure and presentation between stories makes it extremely difficult to apply the same route to all the various possibilities.
Because short stories are, by definition, not lengthy, they don't have the luxury of long explanations or introductions of setting and characters. Most short stories tend to present important elements fairly succinctly, which should help with the isolation and identification of those items. When looking for the theme, the reader needs to be searching for the common factor - the idea or motif or emotion that recurs throughout the story. If the reader can find that base point to which all the action keeps returning, the reader has found the theme of the story.
In order to find a theme in a short story, you must examine what you think the author wants you to learn. Because short stories are small in lenght, it is usually easier to fing the moral of the story wrapped around the rising action or climax. Since both of these areas of the plot revolve around the conflict of the story, examine the conflict and decide what the connection between the story and real life might be.