During the presentation
During the presentation, look for cues that will help you gauge audience reaction. Body language can indicate interest or boredom: Are listeners nodding their heads in agreement or sitting with their arms crossed? The questions they ask can help you identify areas of your presentation that need clarification.
If possible, have another person in the audience take notes on audience reaction for you. You can make notes on those issues immediately after the presentation while it is still fresh in your mind. In many situations it is appropriate to directly ask the audience how it's going. For example, "Does this make sense so far?"