如何告诉angularjs表单输入值已更改

I built a sign up from using angularjs. The submitting process is OK. Then I want to make a validation of checking if the email is already registered. If exist, show the error message informing that the email exists. The problem I face is when user changes the email address, the submitted email is still the previous one. How to tell angular that the input value has changed and submit the newly inserted values.I built a sign up from using angularjs. The sub