Number one, the audience will tell you. Perhaps not in words. But, after standing in front of an audience for 20 minutes, you should have a pretty good idea of what worked and what didn't. There are other more formal ways to gauge how you've performed, such as handing out evaluation forms and asking the host organization how you did. Of course, the best indicator you have scored a bull's eye occurs when the organization asks you to return.
Presentation show some energy, some spark.
Feedback Learn by asking how you've done. It's what we stress in our public speech training workshops at Barks Communications.