How to check if fullscreen on or off?

Hmm, this is another question that google would solve, if you just searched for check fullscreen Unity.

Anyway, your code is mostly correct, Look at Screen.fullScreen in the docs, then you just have to use == instead of =:

 if(screen.fullScreen == true)
 {
     print("fullscreen is on"); 
 }
 else 
 { 
    print("fullscreen is off"); 
 }

Although, here's a pro-tip for you. If you're ever checking to see if something == true in an if statement, you can just put that variable.

ie if(Screen.fullScreen) is just a cleaner way to write if(Screen.fullScreen == true)