what's the solution to detect data changed in textbox field?

 When postback a web form, want to know which textbox field data has been changed.
Javascript is not enough. For example. data in textbox1 is Value1.  Then user maybe type in this box and chang it back to Value1. So the value for this box is not change. If onchange event for this box, can know if the data is changed.
what's the best solution to detect the real change data when postback?

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