Locating pipes in garden
In typical UK homes, critical infrastructure, like electrical power, gas, water and sewage should be deep enough that there is no danger of hitting it with a garden fork from the surface. About two or three feet would be typical I believe. You can check the depth of your sewer pipes at the manhole located near your home.

For example, according to Anglian Water


  There is a minimum and maximum depth at which service pipes should be laid:
  
  Minimum depth is 750 mm
     Maximum depth is 1350 mm


750mm is about 2½ feet.

In other countries the situation may differ of course. If you have a very old home or unusual terrain you might need to take more care.

In addition, I don't think you could realistically puncture any of these with a manually operated garden fork. I assume you have not sharpened the tines and are not using a club hammer to force the fork into your lawn.

Tracing underground services from the surface needs expensive specialised equipment (random example, another) which can be hired (random example).