question on Security: New user creation

Hey folks,

I have a question regarding the new user creation form when using hte security module, on entering an email address it pops up with a green tick next to the email field. Now I cant seem to find where or how that is (the tick) displayed. there are no events within the form at all… so where does this come from?
How would I achieve the same result on another form with an email field?

Its a property of email type field