Application.CanStreamedLevelBeLoaded  

// Check if level at index 1 can be loaded.
// If it can be loaded then load it.
function Update() {
if (Application.CanStreamedLevelBeLoaded(1)) {
Application.LoadLevel(1);
}
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Update() {
if (Application.CanStreamedLevelBeLoaded(1))
Application.LoadLevel(1);

}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Update():
if Application.CanStreamedLevelBeLoaded(1):
Application.LoadLevel(1)

// Check if "Level1" can be loaded, if it can be loaded then load it.
function Update() {
if (Application.CanStreamedLevelBeLoaded("Level1")) {
Application.LoadLevel("Level1");
}
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Update() {
if (Application.CanStreamedLevelBeLoaded("Level1"))
Application.LoadLevel("Level1");

}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Update():
if Application.CanStreamedLevelBeLoaded('Level1'):
Application.LoadLevel('Level1')