Sir82, I've actually done that before! I would say something like: "Glad you enjoyed it. Was there something that stood out to you? I'd really like to improve the talk and highlight areas that people really found interesting". Usually, the response back was along the line of they liked the general delivery of the talk. I had one person actually mention a specific point to me.
Even though I was all in at the time, I knew that most people weren't really listening!