This Underground Cable Tracer project could come in handy next time you are installing a fence. You could scout out the underground hazards yourself to plan where the posts will go. I would recommend that you call to have the utility company confirm your findings though since the equipment they have is very sensitive and they also have plans showing where the cables lie.

"The Cable Tracer is a device to detect underground cables. It was designed and tested on un-energized electric power cables but could be used on any underground cable. A 125 Khz signal is injected into the underground cable from a Signal Generator. The output voltage level of the Signal Generator will effect how deep of a cable can be detected. The Generator used in my setup was a HP 3310B which has up to a 30 volt output. I was able to sense cables buried around 2 feet with only a 10 volt output. The Circuit Breaker was turned OFF in the Panel box and the Signal Generator output was connected to the cable. The pickup coil that I used was an old Degaussing coil from a small CRT monitor. The coil dimensions were 8 x 7 inches. The Pickup coil is extremely directional which even allowed for detecting the curving of the cable. The underground cable that I was searching for was about 200 feet long and consisted of 3 conductor 00 wire. I was even able to locate a cable break by a substantial decrease in signal level as I traced along the cable. The meter was very useful in indicating the direction by seeing the minor level changes as the direction of the coil was rotated."

Via: YourITronics