html5 form validation
it seems that the pop up itself cannot be verified in a typical UI-element wise manner.
However, that popup (or rather the text value of the popup, seems to be captured in a property of the textfield itself called "validationMessage" where it reads "Please fill out this field." when the textfield is empty
and instead returns an empty string when there is some input in the textfield itself
you could look to verify that property via a checkpoint, to make sure that it either reads the intended value, or holds an empty string value instead.