Checksum of .aspx/.ascx

Question

Hi,
I have a requirement where i have to show a javascript alert when you move from tone .net page to another .net page but condition is that at least one control's value on that page should have changed.
Actually, there are more than one page in tabs format. You are on a tab1. You edit the page and make some changes and now you click on tab2 but before moving to tab2 you want to show a validation message. Validation message should appear only when there is  any change on the page.
I have heard about checksum but don't know how to use it and this will work on not.
Any help would be appreciated.
Thanks in advance