How can I avoid wiring when drilling into walls?
Wires 'NORMALLY' travel down into the socket; however if the sockets are daisy chained (and an outlet most likely will be) in other words you will have more than one socket on a circuit; you will have wires traveling horizontally to the next outlet. I said normally - because you never know how those wires were stretched across the space etc.. unless you physically saw them before the dry wall was fastened. 


You can use a stud finder with a wire detector to determine where wires are (~ $40) - to some extent they work. 
There are non-contact voltage detectors that might find it for you.
Then Of course there is a product advertised on TV that will allow your android or iPhone to see through the wall .. the wires, the pipes and whatever. While not cheap it is probably the best tool you could use.


1: Smartphone-sensor-lets-you-peer-right-through-wall

2: 3d-sensor-walabot-vayyar