How to check if Mouse is touching GUI Texture

How do you check if the mouse is touching a GUI texture in scripting (I am trying to use an "if" statement, not a function). As in:

function Update () { 
   if (?){ 
      camera1.enabled = false; 
      camera2.enabled = true; 
   } 
   if (?){ 
      camera1.enabled = true; 
      camera2.enabled = false; 
   }     
}