Put a button beside the textbox to check the availability of the username.
Use Ajax, It will not make partial postback so no flicker on screen. Give a seamless view, that your page has not made any round trip.
and ofcourse it is not possible to check it without sending to database as you are storing the username or email to database only.
You can also implement this feature on textbox textchange event. But it will make unnecessary round trips.
So Best of luck and hope you will get my point....