Gizmos.DrawRay

Description

Draws a ray starting at from to from + direction.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    void OnDrawGizmosSelected()
    {
        // Draws a 5 unit long red line in front of the object
        Gizmos.color = Color.red;
        Vector3 direction = transform.TransformDirection(Vector3.forward) * 5;
        Gizmos.DrawRay(transform.position, direction);
    }
}