Here's a very simple, but practical circuit, which is used to check the type of LAN cables (straight or cross) as well as possible faults. So we use a unit that has 8 outputs, each one of which produces a pulse successivly. Only one output can be high at any time. Then we use two rj45 connectors and we apply the pulses to the 8 pins of one connector (A) wnd we connect LEDs at the pins of the other connector (B). If we connect a straight LAN cable, we notice that the LEDs glow one by one successively. If a wire is broken, the coresponding LED will not glow. Just watch the LEDs. If we connect a cross wire, then the order of LEDs glowing changes to 1,2,7,4,5,8,3,6. So we can laber the LEDs in that order, so that we can watch easily.
If both ends of the cable are not close enough to be pluged onto the curcuit, we can use an remote board, which has only one rj-45 connector and 8 LEDs. Then plug one end of the cable to connector A on main board and connect the other end to the remote unit. Notice that no common wire is needed for the remote unit.
I have included an alternative circuit instead of using classic ics, I use the AVR ATtiny2313, but any AVR controller can be easily adapted. See photo and diagram.