Jim, when you do get one located, take the time to photograph the marks, being careful to align the picture angle with permanent landmarks in the background. Then draw it up on a map, with dimensions from building corners and other permanent points. Then file the pics and the map away for future use. You (or whoever comes behind you) will be glad you did. Don't you wish the previous owner had done that?
For a new line - I like to stretch a string line from point to point, paint a line, then trench the paint mark. Sometimes there's obstacles that won't allow you to do that, but I've got several buried water and electric lines here on the farm that go point to point like that. Never have to wonder where they are, and it only takes 5 minutes to lay it out on day one. . .