Unity Script Reference – GUIUtility.GUIToScreenPoint

// Check the difference between the mouse position (Screen) and
// the converted GUI positions because of the group.

function OnGUI () {
var screenPos : Vector2 = Event.current.mousePosition;
GUI.BeginGroup (Rect (10,10,100,100));
var convertedGUIPos : Vector2 = GUIUtility.ScreenToGUIPoint(screenPos);
GUI.EndGroup ();
Debug.Log("Screen: " + screenPos + " GUI: " + convertedGUIPos);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnGUI() {
Vector2 screenPos = Event.current.mousePosition;
GUI.BeginGroup(new Rect(10, 10, 100, 100));
Vector2 convertedGUIPos = GUIUtility.ScreenToGUIPoint(screenPos);
GUI.EndGroup();
Debug.Log("Screen: " + screenPos + " GUI: " + convertedGUIPos);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnGUI():
screenPos as Vector2 = Event.current.mousePosition
GUI.BeginGroup(Rect(10, 10, 100, 100))
convertedGUIPos as Vector2 = GUIUtility.ScreenToGUIPoint(screenPos)
GUI.EndGroup()
Debug.Log(((('Screen: ' + screenPos) + ' GUI: ') + convertedGUIPos))