How do I determine the state of a toggled element?

You can determine whether an element is collapsed or not by using the :visible and :hidden selectors.

var isVisible = $( "#myDiv" ).is( ":visible" );
 
var isHidden = $( "#myDiv" ).is( ":hidden" );

If you're simply acting on an element based on its visibility, just include :visible or :hidden in the selector expression. For example:

$( "#myDiv:visible" ).animate({
 
    left: "+=200px"
 
}, "slow" );