There are different options to change
1. Client Side
This can be done by using the event onKeyUp. Or you can also do the same by using setInterval, and checking prediodically.
2. Server side
Keep the value in the session variable. On Post back check the check value of text box with session
For details check the articles
http://shahed-kazi.blogspot.com/2009/08/check-if-textbox-changed-using.htmlhttp://forums.asp.net/t/1460964.aspxhttp://stackoverflow.com/questions/1481152/jquery-how-to-detect-a-textboxs-content-has-changedHope this will help you :-)