Zoho Creator only validates numeric fields on submit. How to validate numeric field on user input not validate!!!

Zoho Creator only validates numeric fields on submit.

I have a series of numeric fields which a script calculates on user input to display the total in another field.

A user can enter non numeric information into a numeric field and the on user input calc displays an error on the screen.

How can you validate if the input into the numeric field is numeric before doing the calculation?

Thanks.....