How I understand my ISP use squid

Showing the user's real IP using http headers does not make the proxy itself undetectable to the other side. It depends on which data the other side uses to determine the IP address requesting it. I'm pretty sure that proxy-detection sites make it clear that when the connecting IP address (tcp level) does not match the requesting IP address (http level) that your requests are being proxied.
 
Dont know way how to compare ip from "tcp level" and "http level"... Http request give server hostname only, not ip isnot?

You can install firebug addon to firefox. Goin to a site, you can see http response headers(right bottom icon) - if there is directive saying "Proxy-status miss" or like this, this must be 'clear' connection.
 
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