how to know the textchanged event in if condition

If you want to fire an event other than textchanged when the text is changed in the textbox , then do like this

<asp:textbox runat="server" id="tb1" xmlns:asp="#unknown"></asp:textbox>

Add onblur event to the textbox in code behind like this

tb1.Attributes.Add("onblur", "javascript:tbFn(this);");

function tbFn(tb) {
       var text = tb.value;
   }

where the variable text contains the entered value.
If you want to compare with its previous values, store it in some session and compare the values.

Hope this helps...