There is a type of packet in the stream that can have an entry called a Network Information Table or NIT. This should contain information about other transponders and data rates associated with that 'network'.
During a Network Scan or Network Search( different receivers call it different things), it will read this table and then go and look for other transponders in the table for additional channels.
Not used here in NA very much though. Used to be used on 97W some, not sure if it is anymore or not.