How do I correct my bounds in 2.5D?

I have a 2.5D game (2D game with 3D objects), and I need to detect when the bounds of my 3D objects have reached the edges of the screen. I'm having trouble figuring this out. How do I detect if the outer edge of my 3D object is at the edge of the screen?

 

Probably set a variable using Screentoworldpoint(Screen.width,screen.height) for the max and screentoworldpoint( 0,0) for the min. (Or something very similar to that) Then on your object just check against this value (make sure you take into account object width) on all 4 sides.