how to find if specific element exists in html

Using Pure Javascript, you can check if element exists as below:

var element =  document.getElementById('elementId');
if (typeof(element) != 'undefined' && element != null)
{
  // Yes Element Exists.
}

OR

You can use jQuery function length() to check if there any element with associated class/div, with code like 

if ($("#ElementId").length == 0) {
        //Element does not exist, if value is greater than 0, element exists
   //.length() gives the size of number of elements associated with particular id/class in html
    }

Check this url fore more details

For your code, you can use this:

//to Stick Ads while scroll on Desktop
    var StickHead= 0;
    
    if ($("#StickHead").length != 0) {
        StickHead= $('#StickHead').offset().top;       
    }
   
    $(window).scroll(function () {     
       //in your code i think you are pointing 'StickHead' instead of 'stickyHeaderTop'  
         if ($(window).scrollTop() > StickHead) {       
            $('#StickHead').css({ position: 'fixed', top: '50px' });           
        } else {           
            $('#StickHead').css({ position: 'static', top: '0px' });           
        }
    });

it should work, let me know, thank you

To check if element exists

Using jQuery:

if( $('#ElementToFind').length )         // use Id as selector
{
     // yes element exists
}


if( $('.ElementToFind').length )         // use Class as selector
{
     // yes element exists
}

You can also use JQuery Array Notation to check if element exists

var elemnt = $('.ElementClass')[0];
if (elemnt ) {
    console.log('element found', elemnt );
}

Using JavaScript:

if(document.querySelectorAll("#element ").length){
   //element Exists
}

OR

if (document.getElementById('ElementId') instanceof Object){
    // Element found, do something here
}