The cheap commercial ones just have 2 banks of common style connectors. Then 2 rows of LED's. One row indicates voltage going IN to the cable and the other comming out. Then a push button lets you step though applying voltage to each connection pin and you watch and see if the LEDs track each other. Simple and effective.
Thanks for the prompt reply. It is a very good idea to do this way. What happen if I need to do different types of cables and might forget which wire goes to which connection.