The checkbox would be the easiest solution, add the checkbox to the registration form, set the checked value to 1, and add server validation to ensure that it has been checked.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.