Theme teaches a life lesson. It is what the author or the narrator wants the reader to learn. It is always a complete sentence. Choosing a topic first and "extending it into a life lesson" helps identify theme more easily.
The theme can also be found through the plot. Here is a plot of the 3 Little Pigs outline. Based on the conflict, I can identify a theme. Taking your your time in the beginning and doing a little extra work, can be helpful in the end.