How would I check if an element is hidden in jQuery?

You can use use .is(":hidden") or .is(":visible").

Example:

<div id="div1" style="display:none">
  <div id="div2" style="display:block">Div2</div>
</div>

if ( $(element).css('display') == 'none' || $(element).css("visibility") == "hidden"){
    // 'element' is hidden
}