Screen.fullScreen

Is the game running fullscreen?

It is possible to toggle fullscreen mode by changing this property:

JavaScripts

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
Screen.fullScreen = !Screen.fullScreen;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
Screen.fullScreen = (not Screen.fullScreen)

A fullscreen switch does not happen immediately; it will actually happen when the current frame is finished.