checking to see if userid or email address already exists

I am trying to figure out how to check if the user id or email already exists in my system when the tries to sign up. I am using default asp.net sign up functionality. when I look at the create button on register.aspx page I see this:
 

                        <p class="submitButton">
                            <asp:Button ID="CreateUserButton" runat="server" CommandName="MoveNext" Text="Create User"
                                 ValidationGroup="RegisterUserValidationGroup"/>
       &nbs ...