You can transform the 3D coordinates into Screen coordinates and then assign this to a GUI.Label or whatever you want:
function Update () {
var screenPos = camera.WorldToScreenPoint (target.position);
print ("target is " + screenPos.x + " pixels from the left");
}
Keep in mind that the Screen Positions start at the left/bottom corner but the GUI starts at the left/top corner so you will have to use something like: