Asking a couple of questions at the end of the event should give you enough information to work out what they got from the event. You could do this as part of a survey, ask people to write their answers on sticky notes that they add to a wall/table on their way out of the event or verbally.
Questions to ask include:
What did you enjoy most today?
Is there anything you have learnt today?
What else could we offer at future events?
Write down the one thing from your conversation that has really impacted you.
If we run the Human Library again, what should we do differently?
