Has de usar esto:
strVariable.lastIndexOf(substring, startindex)
Código:
<html>
<head>
<title> New Document </title>
<script language="Javascript">
<!--//
function validaEmail(theFormField){
enviar=true;
valor = theFormField.value;
if(valor.lastIndexOf("`",0)<0){enviar=false;}
if(valor.lastIndexOf("´",0)<0){enviar=false;}
if(enviar){
alert('enviado');
}else{
alert('el mail contiene un acento');
}
}
//-->
</script>
</head>
<body>
<form name="tu_form">
<input type="text" name="email"><br><br>
<input type="button" value="enviar" onclick="javascript:validaEmail(document.tu_form.email);">
</form>
</body>
</html>