How to retrieve control's value from ViewState

[epigram]

I'm responding to a button click event on an asp.net web form. I then need
to retrieve the value from a TextBox control and I want to compare it
against the control's previous value to see if it has changed. How can I
retrieve a control's previous value from the ViewState? I know that I could
save the control's previous value in a session variable, reretrieve it's
value from the db, etc. But it would appear the control's previous value
would be in the ViewState since that is what ASP.NET uses to fire events
such as TextChanged.

Thanks!