They don't. They may or may not use MAC address registration in your area. (AT&T did when they were our ISP, when Comcast took over they seemed to stop doing it.) The MAC registration requires that you tell Comcast the MAC of whichever computer or router is being connected to the cable modem. If they don't use it anymore, then you can connect anything. If they do, then you either have to call them anytime you switch machines, or always use a router, or use the cloning capability of some OSes and routers to always fake the same MAC.
The only way to figure it out would be to ask them (if you don't have service yet) or to see if your system has PPPoE configured on it when they installed it.