var select;
select = document.getElementById("foo");
if(select.value == ""){
   document.getElementById("bar").style = "border: 1px solid red;"
}

<span id="bar">
<select id="foo" name="id[1]">
  <option value="">Bitte wählen</option>
  <option value="foo">bar</option>
</select>
</span>

<form action="" method="post" onsubmit="return CheckForm( );">

function CheckForm( )
{
  var select = document.getElementById( 'foo' );
  var error  = false;  

  if ( select.value == '' )
  {
     document.getElementById( 'bar' ).style = "border: 1px solit red;"
     error = true;
  }
  
  /* Andere Formulardaten prüfen... */

  if ( error )
    return false; /* So wird das formular nicht abgesendet */
  else
    return true;
}