Plumbers have ground penetrating radar dont know if it sees roots . Anyway its expensive to use several hundred dollars a hour . Look up Doodle Buggers and oil . Hardly practical and overkill on a huge scale ,but it does answer your question "How Too " Poor mans answer is to do a little digging  and see where it goes.

Dig ?

what you mean when you want to trace a root of tree?

it depends on the quality and deep of soil in any part of your ground.

As Above so Below/Mirror Image