Mathf.Infinity  

// Casts a ray from (0,0,0) towards (0,0,1) to the infinity and prints a message
// if any object has touched the ray.
// To test it, just place any object and intersect it with the white drawn line

function Update () {
// shows the line that follows the ray.
Debug.DrawLine(Vector3.zero, Vector3.forward * 100);
if (Physics.Raycast (Vector3.zero, Vector3.forward, Mathf.Infinity)) {
print ("There is something in front of the object!");
}
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Update() {
Debug.DrawLine(Vector3.zero, Vector3.forward * 100);
if (Physics.Raycast(Vector3.zero, Vector3.forward, Mathf.Infinity))
print("There is something in front of the object!");

}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Update():
Debug.DrawLine(Vector3.zero, (Vector3.forward * 100))
if Physics.Raycast(Vector3.zero, Vector3.forward, Mathf.Infinity):
print('There is something in front of the object!')