How to get the property(id, CSS, xPath) of a popup

I have a disturbing popup that breaks automation test. It is client side validation popup appended to a textfield that only takes number, if I input a text and click the submit button, the popup will suddenly appear and stop the test. How do I overcome this? Appreciate. It is the one having a yellowish bos with exclamation mark, that reads "Please enter a number". When I viewed the source of the page, I found nothing to reference on this popup.