Can cable company tell if I receive TV signal?

About a month ago, I signed up for Comcast cable broadband. Broadband
is all I ordered,  all I am billed for and that is all I paid for.

However, when the installer hooked up the broadband signal, I
subsequently found out that he inadvertantly also hooked up the cable
TV signal. 

Questions: 
1. if I split the cable and route one split to a TV, other than
checking the connection itself, how can Comcast tell that I am
receiving the TV signal in addition to the broadband signal?  (All
connections in our neighborhood take place atop the utility poles out
in the street so I cannot be blamed for illegally hooking it up.)

2. If I do this, will my broadband signal be degraded materially?