I worked for a major cable provider, not on the cable end, but on the computer programming end, that company has since been bought and sold and bought and acquired and spun off and my friends that are still there spend more time in "corporate branding" meetings than they do actually sitting at their desks. But at least they get new t-shirts and coffee mugs about every 6 months /ubbthreads/images/graemlins/grin.gif
I used to like to chat with the folks that were just getting the cable modem systems up and running and figuring them out at the time and asked similar questions. This was enough years ago now that technology could have changed though. They can send s signal down the lines that bounces back from endpoints and so they can sometimes tell how many boxes are connected. but this can be fudged up by amplifiers, traps, even splices and especially bad cables or cables with breaks in the insulation or kinks. (thats actually what the technology was designed to find was where in a length of cable the fault was) So it's not very accurate.
However, it's very easy for them to drive around and just see if there is evidence of tampering in their boxes or that the box connections match up with what the billing system says should be connected. Which is probably how they go about doing it. Just pick a neighborhood and with paperwork in one hand check all the wire connections with the other. And they really don't have to do more than a few token audits around town to get a lot of people worried enough to call in and start paying for it /ubbthreads/images/graemlins/smile.gif
As the technology continues to advance and there are more 2-way things connected that will report their presence back to the CO it will be easier for them to tell and harder for you to steal it...