How proxy is detected?

Under some members number of posts ( like mine ), could be seen "proxy detected" , how a web site or AO can detect if the visitor is using proxy?
And why AO are mention it?
BTW, I am not using any proxy now, but still is appears.

You proxy is detected because it's a web (chacheing) proxy. If you were using an anonymous proxy or socks, I don't think that it would be detected. The "proxy detected" message is another AO feature. AO's constantly being updated and changed so be sure to read the site updates and new features when JP posts em.. ;)

Thanks. but how AO can detect it?

Maybe it looks for the HTTP header "Via" which many proxies add.

My ISP has a transparent proxy, but I believe it adds "Via" headers.

You can detect proxies via some clever PHP scripting, usually by examining the headers that proxies send out. Of course, not all proxies send this information for whatever reason, I'm behind a proxy at the moment but you won't see 'proxy detected' under my profile/username.

This script at Overdose explains a simple way to detect proxies. It's not foolproof by any means, but I think it's similiar to what JP uses.