<div id="el1"></div>

<script type="text/javascript">
function isElem(el){
   return (typeof el=='string'?document.getElementById(el):el)!= null?true:false
}
alert(isElem('el1')); // true
alert(isElem(document.getElementById('el1'))) // true
alert(isElem('el2')) // false
alert(isElem(document.getElementById('el2'))) // false
</script>