Dynamic form field evalutation

Hello

I want to evaluate user form field input 'as she types' versus the database to check if the value already exists (ex. username).
If this is the case I want to inform her interactively below the form field. I guess you have a grasp of what I mean.

How can I accomplish that?
Any code examples?

Any help as always very appreciated.

Cheers

Gee

The only absolute knowledge attainable by man is that life is meaningless. (Tolstoi)