The easiest way to locate underground services is to dig a small hole with an excavator. They will hit the services every time :doh:
Alternatively you can probe using a small metal rod. Push the rod into the ground till it contacts the service. It is easy to tell when you hit a pipe or conduit and can work well as long as the service isn't too deep. A 'T' handle on the top gives you a bit of purchase and you can twist back and forth to get through the harder bits.