Super Chexx uses an array of magnetic reed sensors to see the puck go into the net. As the puck travels over the top the magnetic field from the pucks magnet moves the contacts inside the reed switches connecting the circuit sending the signal to the main board. You can the signal with a digital multi meter, there will be 5vdc between the two wires and the voltage will drop to 0 when the puck travels across. If you don't have a multi meter you can emulate the signal to test the main board by following the guide attached to this article.
The reed switches are sealed inside the glass tubes which are made from glass once it breaks it will no longer function!