How to check that certain element does not exist ?

How to check that certain element does not exist ?

I am dynamically creating CheckBoxes DropDoenlist with certain set of values of not all of then are created any time.
But in Script i want to get sure whether that element has been created or not. So how can i check for radio button list and dropdown list.
Below is the code for my dynamic list creation:

  1. sbp1+="<select id='locationOption' class='reg_select'>";
                      sbp1+="<option>Location one</option>";
                      sbp1+="<option>Location two</option>";
                      sbp1+="<option>Location three</option>";
                      sbp1+="<option>Location four</option>";
                      sbp1+="</select>





  3.  $("div#thrustAreaOptions").append(sbp1);

On certain situation this code is executed:
  1.  $("div#thrustAreaOptions").empty();
But in main javaScript function i want to make sure whether that's element exist or not.

Thanks