check if textbox value in a gridview is changed

Hi,
I have a gridView and has an edit function to modify the data in a row of the gridview.
When I click edit, then the row shows some textboxes in the row in the different fields. current the editing function is working but I have a field shows editedby. for example, if userA logon the page and made some changes, then that field shows UserA made changes for that row. Then userB logon, but if userB did not make any changes and just clicked edit and then save, then the editedby changed to userB which is not right as userB did not make any change. Therefore I need to check those textboxes in the row of gridview and see if there are some change after click save button. 
1. if either textbox value has changed, then the editedby will change to current login user
2. if nothing changed, then the editedby field remains by the user who made changes not current login user.

Question: How can I check if the textbox value has changed or ...