Testing apparatus for the electrical system of a truck, comprising - a plug for insertion into a trailer socket of the truck, which comprises plug contacts (1 to 7) for contacting socket contacts of the trailer socket, - a housing to which the plug is attached, in which a testing device is mounted and on which at least one optical signal transmitter (21 to 27) is arranged, - wherein the testing device is connected to the plug contacts (1 to 7) and the at least one signal transmitter (21 to 27) and actuates the at least one signal transmitter (21 to 27) as a function of electrical voltages between the plug contacts (1 to 7), - wherein the testing device activates the at least one signal transmitter (21 to 27) both as a function of whether or not any electrical voltage with respect to a reference potential, which has the correct sign, is present at at least one of the plug contacts (1 to 7), and as a function of whether or not the voltage between the at least one of the plug contacts (1 to 7) and the reference potential reaches a predetermined minimum value, characterized in - that the minimum value of the electrical voltage is specified as required for the safe operation of a trailer to be connected to the truck via the trailer socket, and - that it can be read from the at least one signal transmitter (21 to 27) whether the socket contact assigned to the plug contact (1 to 7) is just not supplying sufficient voltage or is even basically incorrectly contacted, wherein the at least one signal transmitter (21 to 27) is designed to actively output at least two different signals and/or has at least two different signaling elements which can be activated, and - that the testing device activates the at least one signal transmitter (21 to 27) as a function of whether or not a current flows via at least one of the plug contacts (1 to 7), which is assigned to one of the socket contacts for an electrical earth, when a predetermined test voltage is applied, which current reaches a predetermined minimum current value for the one of the socket contacts. - a plug for insertion into a trailer socket of the truck, which comprises plug contacts (1 to 7) for contacting socket contacts of the trailer socket, - a housing to which the plug is attached, in which a testing device is mounted and on which at least one optical signal transmitter (21 to 27) is arranged, - wherein the testing device is connected to the plug contacts (1 to 7) and the at least one signal transmitter (21 to 27) and actuates the at least one signal transmitter (21 to 27) as a function of electrical voltages between the plug contacts (1 to 7), - wherein the testing device activates the at least one signal transmitter (21 to 27) both as a function of whether or not any electrical voltage with respect to a reference potential, which has the correct sign, is present at at least one of the plug contacts (1 to 7), and as a function of whether or not the voltage between the at least one of the plug contacts (1 to 7) and the reference potential reaches a predetermined minimum value, characterized in - that the minimum value of the electrical voltage is specified as required for the safe operation of a trailer to be connected to the truck via the trailer socket, and - that it can be read from the at least one signal transmitter (21 to 27) whether the socket contact assigned to the plug contact (1 to 7) is just not supplying sufficient voltage or is even basically incorrectly contacted, wherein the at least one signal transmitter (21 to 27) is designed to actively output at least two different signals and/or has at least two different signaling elements which can be activated, and - that the testing device activates the at least one signal transmitter (21 to 27) as a function of whether or not a current flows via at least one of the plug contacts (1 to 7), which is assigned to one of the socket contacts for an electrical earth, when a predetermined test voltage is applied, which current reaches a predetermined minimum current value for the one of the socket contacts.