<input value="600.00" type="checkbox" name="cover_letter" id="nat_cover_letter">

<script>

$('#nat_cover_letter').change(function() {
if($(this).is(":checked")) {
alert('checked');
}
else
{
alert('unchecked');
}
});

</script>

Or you can check at run time as well like this :

<script>

if($("#nat_cover_letter").prop('checked') == true)
{
alert('checked');
}
else
{
alert('unchecked');
}

</script>