The data sheets for those sensors are as near to worthless as I can imagine! http://www.cdiweb.com/ProductDetail/OPT43035-Madison-Company/412112/#.Ukul3FNu3DA
If you are seriously concerned about device failures, you will need a lot more information. I suggest to contact Madison's technical support to learn, for example, the typical current draw and the load (current and voltage) specifications for the output. Perhaps they have technical info on reliability and functionality checks as well. I'm assuming here that these are not simple switches, but have some electronics inside. Otherwise, why would there be a spec for 5-12 V?
Presumably, the sensor draws some power regardless of the output state, so if you power the sensor from the arduino side you can at least check that it is drawing the nominal amount of current. That would verify power and ground leads as well as nominal functionality. If the current draw is around 5-10 mA, you could use an optocoupler to verify that the sensor is drawing current. For example, place the LED side of the optocoupler in series with the sensor positive supply lead.
However, wire breaks would not be the only failure mode. You could also have shorts to ground on the positive power lead and on the signal output lead. Without more info there is no easy way to check the latter. Perhaps the best bet is to use high quality 3-conductor cable and place sensors in pairs.