You can't see or hear yourself the way your audience does.
You can't catch all your crutchwords like "ah" and "so". That means you can't spot ways to improve. For instance, you may use more crutchwords in some circumstances.
You can't see yourself or your gestures. Do your facial expressions look natural? Do you talk more to one side of the room?
SurprisesWhen you're speaking, you know what you intended to say and do. You notice the gaps while you’re speaking. If only practiced more! If only you hadn’t messed up that quotation!
Audiences are forgiving. They only know what they see and perceive. You're probably better than you think. How would you know?
Testing 1-2-3Always record yourself. You can then review your performance and make changes. You can also see how you're improving speech by speech.
You can record audio, video or both.
Audio is easiest. You can use your smartphone or a separate recorder. You can leave your recorder at your seat if you want to record from the audience's vantage point. The quality may suffer. Another way is to bring your recorder to the front of the room with you. Place it on or near the podium.
Video is more effective but requires a video camera and setup. Your Toastmasters club may record speeches for you. That's effortless for you but you may still want to use your own equipment. If you prefer audio, you can probably record that alone with your video camera.
You can record both audio and video. That gives you backups but you may get overwhelmed worrying about all the equipment. Simpler is often better because less can go wrong.