Tutorial Check if Element Exists

if ($('#myElement').length > 0) {
// it exists
}
Or to make it a fancy function with a callback:
// Tiny jQuery Plugin
// by Chris Goodchild
$.fn.exists = function(callback) {
var args = [].slice.call(arguments, 1);

if (this.length) {
callback.call(this, args);
}

return this;
};

// Usage
$('div.test').exists(function() {
this.append('

I exist!

');
});