> When I open a webpage and view its source, I wonder
> how to open and view form action url from a user's
> aspect.
> 
> As in the following example, how to open and view
> the file "/destination_url" ?> 
> <FORM ACTION="/destination_url" METHOD=GET>
> <INPUT TYPE=submit NAME=foo VALUE="Go to
> destination">
> </FORM>

if the current page is some.site.root/path/page.htm then
action="/destination_url" url is some.site.root/destination_url
If action was just "destination_url" the url would be 
some.site.root/path/destination_url

But pasting that into your browser may not show you anything useful since your
not sending the rest of the form to that url.