How to read previous page URL in my application?

Hi,
   Is it possible to read previous page url if the page is not opened using response.redirect or server.transfer.
   My requirement is,I have an application say App1 and this App1 is called from some other application say App2.I dont have control over APP2 to change the code.App1 can be accessed in many wyas.It can be accessed as a standalone application or from someother applications.Now I want to read from which application my APP1 is accessed.App1 is accessed from other aplications using <a href="http://xyz/ABC/..../home.aspx">.
   Is there any possibility to read the url of previously accessed page??Can anyone please help me.....
   Any help is much appreciated.
Thanks in advance
Sri
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