It's actually really easy today for the vast majority of people with modern "cable" in the US. The reason why you need a cable box for each TV isn't just for de-scrambling the signal, it's also so we measure what channel you watched, when, and for how long. Go look into each cable box... it's basically a computer just like your modern Samsung TV, and like all computers it has "telemetry" baked in. We record telemetry regardless if it's live or DVR -- that's why the DVR is integrated for most consumers today.
All the major ad buyers know their "reach" per placement (the individual ad being broadcast during programming) pretty accurately. What's harder still is which people actually saw the ad, but we're getting closer.
We already know who you are as a subscriber (you had to provide us a SSN, phone number, name, and mailing address to get an account), so we literally know your household.
The only missing piece is which member of the household watches each show. There is some work to correlate internet traffic to infer which person was home at the time the ad played, but it's been less than helpful during prime time broadcasts when the whole household is home anyway.
We're working on it.
Anon because broadcasting all this tracking isn't good for my career.