Event.delta

Description

The relative movement of the mouse compared to last event.

Used in EventType.MouseMove, EventType.MouseDrag, EventType.ScrollWheel events.

See Also: Event.mousePosition.

// 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 ExampleClass : MonoBehaviour {
    void OnGUI() {
        Event e = Event.current;
        if (e.isMouse)
            Debug.Log(e.delta);
        
    }
}