How do I detect the flags set for controls with values changed upon posback?

Let's take the following TextBox Control.

<asp:TextBox ID="txt1" runat="server" AutoPostback="True" />

With AutoPostBack set true, each time a value is changed, it will do a PostBack and save the value if that is what i want to do.

I want to do something with the value when the form is submitted, and look for the controls where the values have been changed, and then do something.

I could take the value of every control and save it even if it hasn't been changed, but I would rather not do that and only change the values that have.

What is the best way to do that?