A $35 tool like the following will do the trick
http://tinyurl.com/agr9z

Ray wrote:

The previous owner of my home burried some Romex to power up a pool pump and
he ran a branch to a receptacle for a non existent sheded off this to
provide a receptacle to a non existent shed.

I like to do some trenching in my yard to run conduit to a workshop that I
built but want to avoid the existing romex.

Is there a way/device to help me find the burried romex. I tried a metal
detector and after adjusting the sensitivity it still picks up a lot of
stray stuff in my yard and does not give me a clear picture of the location
of the romex.

Thanks,