What is the best way to gather feedback of a guest speaker or presenter?

Keep it simple and ask directed questions of the audience.  If your intention is to make a difference, inspire and teach, then your focus is on finding out what they "got" from the presentation rather than if they liked you, the seating or the catering.  Leave the feedback on the venue to the organiser and make sure that you ask questions to measure the effectiveness of your presentation.  In addition to this, have a more detailed presentation review tool to give to a mentor, presentation buddy or room captain during the presentation.  Use this form in your presentation debrief.