function CheckForm() {
if ((document.Form1.field1.value=="") ||
(document.Form1.field2.value=="") ||
(document.Form1.field3.value==""))
{
alert("Please enter your data correctly! Check if all fields are filled
out. ");
return false;
}
}
//-->
</script>
<form name="Form1" method="post" action="page.asp"
onsubmit="return CheckForm()">
<!-- ... form elements here ... -->
</form>