For the most part on standard cable, if I recall correctly, some people have boxes hooked up to their TV's that record what they are watching. This info is measured, and results in the "Nielson ratings". Has been going on for years. "Sweeps week" is huge for this type of measurement as it helps dictate who or who not is being viewed, thus potentially changing the next season's schedule.
Other measurements are water usage. Flushing toilets at the same time can actually cause a significant drop in reserviors. This method has been translated into "ratings" for significant programs like the Superbowl, World Series, etc to help verify other methods.
In the digital era, however, if you have digital cable I would expect them to be monitoring what you are watching, as it would be significantly easier to send a return feed of info back through the line from your cable box.
So let your woman watch whatever crap she wants (if you are on standard cable).... but she ain't allowed to flush!!
Edit.. FYI, the three responses above came in while I was typing this!!