Event.delta  

// Move the scroll wheel to determine
// the X & Y scrolling amount.
function OnGUI() {
var e : Event = Event.current;
if (e.isMouse) {
Debug.Log(e.delta);
}
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnGUI() {
Event e = Event.current;
if (e.isMouse)
Debug.Log(e.delta);

}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnGUI():
e as Event = Event.current
if e.isMouse:
Debug.Log(e.delta)