Cameras would work, as would AI to know which device identifiers tend to be close to one another. Besides, they wouldn't need or want perfect accuracy. I'm picturing basically something like the British does with TV licenses where they more or less assume you're guilty and then badger you into proving you're not. You have some friends over for the game, then you get a bill in the mail that says something like "This address has been linked to a broadcast without express, written consent from the NFL. Please pay $5 or we shut your service off. If you believe this letter is in error feel free to hire an attorney, take a day or two off of work and drive hundreds hundreds of miles to the dispute resolution center you agreed to use for such purposes when you accepted our EULA. We pay them, but who knows? Maybe they'll rule in your favor!"