User created By…?!

Hi experts,

Is there any way to know who has created new users..?
I have created a role named sub-admin and given it permission to create only users(of other roles like editor, subscriber).

Now if there are 3 users of role sub-admin. How do i get to know which sub-admin created which new user(of other roles like editor, subscriber)..?

Thanks in advance…

Vish