Great question there and I never thought about going that granular, and the thought of doing it is a bit daunting so I hope we don't have to do that! But I don't believe IP Address is a concrete way to prove it still — somebody can be logging into his/her gmail account from different cities around the world , so wouldn't the IP address always change depending on what provider they're logging in with?
For proof, my understanding is that as long as we can show the date stamp and clearly show the verbiage of the option they they're opting into ("Sign me up to receive emails from XYZ company").
More thoughts and suggestions welcome out there!