"It's true that we've been posting events on our website calendar for some time now, but it's difficult to ascertain whether the service has helped the program, because when attendees indicate (upon signing in) where they learned of the program it's usually from somewhere else."
The uncertainty is common. It happens to everyone and people usually don't know or remember where they found out about a particular event. Usually people say "I found out on the Internet" and may not say exactly where... In any case, as with all promotions, flyers, etc... If you really want to track what you do, don't rely on people to tell you. The best way to track your efforts is to do or include something specific on each method. This can be simple or can be very elaborate, so you choose the method that works for you.
- In your online event posting, you can simply write: "Mention this event posting on [your website name] to us and receive a free gift at the entrance!". This is nothing new... The gift can be small or whatever you can afford, something inexpensive will suffice. Or you can just write "Please mention this event posting from [your website name] when you attend the event." Again, this explicitly tells folks to remember where they got the info.
- The best method is to use the RSVP feature of your event calendar!!! Encourage people to RSVP right on the event posting. That will give you an realtime report of people who RSVP.
There are other ways to track. What is important and matters most is that you do something and measure the outcome of your efforts. Based on your tracking results you can then evaluate and prioritize what works and what you can do without.