property intrinsicHeight

This property returns the intrinsicHeight of the box is the height that the box requires to to fit its sized content without the need for scaling or scrolling. Example: An image box that has a 75x90 image in it would have an intrinsicHeight of 90. This will always be the case even if a style has specified the box's actual height be 120.

var bx = scene.getBoxById("theBox");
if (bx && ((bx.height > bx.intrinsicHeight) || (bx.width > bx.intrinsicWidth)))
{
    PromptUserForResize(); // <~~ custom user function.
}