怎样用javascript验证文本框输入的数字还是字符？

<Script Language="JavaScript">
<!--

function CheckString() {
var Temp=Num.value.substring(0,Num.value.length-1);
if(event.keyCode<48 || event.keyCode>57)
Num.value=Temp;

}
-->
</Script>
<Input Type=Text OnKeyUp="CheckString()" Name="Num" onpaste="return false;">