Can I use Google earth in surveying to determine the contour line ?

Yes, But it is a bit labourious, yo can pin point different positions and get some same points having same elevations and join them, 

it is better to use surfer or Trimble softwares like WM-Land Form & TBC(Trimble Bussiness Centre Software, in these softwares just put data taken from Hand Hold GPS or FmX Display and Manipulate the Data to get contores line