Since in many cases the control will have some values entered in it, server side validation may not be appropriate. You can use client side handlers to determine the input. The onError
hanler gives additional information as to the cause of the error - whether it was parsing, or range issue. the onValueChanging/onBlur methods are two other options. By using any combination of these options, you can create very flexible validation.
Alternatively, you can use the beforementioned approaches to simply clear the text in the control, which would give you the chance to use a required field validator for example.
I hope one of these options is suitable for you.
All the best,
the Telerik team