Decide how you are going to evaluate the success of your event. How will you compare your actual results with the objectives you set? A short questionnaire for visitors to complete could help you assess the impact of your event and plan for future events. You could organise a prize draw to encourage people to take part.
Ask exhibitors for their views, and if there are measurable ways that their involvement in the event helped them. Can you do some simple research (make a few phone calls) to see if the event raised your profile among the public or other local stakeholders?
One simple measurement of success might be the expansion of your database, by collecting names and addresses of local people who want to know more about learning.