MS Access Forms How to detect if data in field has changed

Hello,
Is there a way to check to see if the value of a combo box or text box has changed?  I have placed my code in the AfterUpdate() event of a control but how can I be sure the user actually changed the data?  For example the user may click on the combo box and actually choose the same selection that it was originally.  In this case I wouldn't want to do anything.  Is there an event I can use that is only trigged if the data actually changes?  

Thanks!