Definitely having some kind of a call to action is going to be the only way to know for sure...
"Clip this coupon" or "mention this ad and save" are good ideas. If you are driving people to a website try getting a vanity URL that supports your brand so that you can see who is hitting that site and what the specific conversion are... even if that site only redirects to another page on your actual website.
Lastly, you could have people call "Sally" (or some other fictitious person who doesn't work for you) when requesting information. That way, you know anyone who asks for "Sally" came from that advertisement and the person who actually answers the phone responds with "Sally is not at her desk at the moment, but I'd be more than happy to help you."
Hope this info is helpful!