Gizmos.DrawRay  

function OnDrawGizmosSelected () {
// Draws a 5 meter long red line in front of the object
Gizmos.color = Color.red;
var direction : Vector3 = transform.TransformDirection (Vector3.forward) * 5;
Gizmos.DrawRay (transform.position, direction);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnDrawGizmosSelected() {
Gizmos.color = Color.red;
Vector3 direction = transform.TransformDirection(Vector3.forward) * 5;
Gizmos.DrawRay(transform.position, direction);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnDrawGizmosSelected():
Gizmos.color = Color.red
direction as Vector3 = (transform.TransformDirection(Vector3.forward) * 5)
Gizmos.DrawRay(transform.position, direction)