jQuery - slideToggle, testing if up or down

Hi all

I have a simple slideToggle that slides a sub menu up and down

Code:
$('a#btn-portfolio').click(function(){
  $('#sub_menu').slideToggle('slow');
});
I also have a .scroll function when the window is scrolled.

If the window is scrolled and the #sub_menu is closed I would like #sub_menu to slide down.

Is there a way of testing if the #sub_menu is open or closed when using slideToggle.

Something like:

Code:
if(!#sub_menu.open){
  #sub_menu.slideDown();
}