Obtain a map of the gas lines near your property from your gas company.
Call your water provider to ask about water mains near your property.
Use a metal detector around your entire property to locate galvanized pipes. If you think you have found a pipe, try to follow it in a certain direction to see if the metal continues or if you have just found an isolated piece of metal.
Use an underground locating tool to search for pipes. These are tools designed to locate underground pipes.
Stick the steel probe into the soil to see if you hit a metal pipe. Note that some soil may be too thick for a probe to penetrate.
Consult your home design plans to see if they indicate waste runoff or water supply. This will give you a specific idea of where pipes would be located.