You can do client side or server side validation. I prefer client side, becuse there's no server trip.
A simple example (you have one DTC textbox and one DTC button for saving):
<script language=JavaScript>
function thisPage_onbeforeserverevent(obj,evnt)
{
if (obj=="btnSave"

{
if (event=="onclick"

{
if (isNaN(document.thisForm.TextBox1.value)==true){
alert("Only numbers please."

;
document.thisForm.TextBox1.focus();
thisPage.cancelEvent=true;
}
}
}
}
</script>
isNaN (is Not a Number) is a JavaScript function that checks if the value is a number. Problems are possible if you're allowing decimal places and your decimal operator is not the dafault one, then your validation should be diffrent.
"Defeat is not the worst of failures. Not to have tried is the true failure."
-George E. Woodberry