Toner probe
A toner probe is a device that helps trace cables through a house or other building. For example, someone tracing a Cat 5 cable may connect a tone generator in one room and use the toner probe to determine the associated cable in another room. This tool is especially helpful when you cannot physically follow the cable because it's behind a wall or you're dealing with many cables.
Once a cable is traced, the person would know the associated room and be able to connect it properly to a networking modem or router. Most toner probes can also indicate if signals are being received over each wire in the cable to help test its connectivity.
The picture shows a network toner probe and generator from Mykit with an RJ-45 for testing Ethernet cable, RJ-11 for testing phone lines, and alligator clamps to test for an open circuit.