Sphere does not keep a record of when a participant clicks on the "I Agree" link on a waiver prior to registering.  Clicking on "I Agree" is required to move forward in the registration process, so when the event site is started with a waiver in place, all participants that make it through the registration process would have had to click on this.

An alternative solution would be to create a custom event field to track this, where you require a user to enter text like "Yes" to a question of whether they agree to the terms of your event.