This won't work for every proxy, high quality proxies won't include any headers indicating that they're forwarding a request from someone else, but some proxies do include this stuff. Put this on a web server of your choice and throw some requests at it to check the quality of the proxies you're routing through. Your actual IP might be leaked more than you think.

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Your IP : 18.208.187.128
Proxy Type : Elite Proxy / No Proxy

/* By G0g0l @ BHW*/
/* This script  is completely opensource. You can use it 
 * ( and abuse it) freely, but please don't blame 
 * me for any consequences :P */

/* Source: http://www.blackhatworld.com/blackhat-seo/php-perl/527166-share-very-simple-script-testing-your-current-proxy-anonymity.html */

/* Can similarly use this, for a quick check: http://www.whatismyip.com */

echo "<b>Your IP : </b>{$_SERVER['REMOTE_ADDR']}<br><b>Proxy Type :</b> ";

if (isset($_SERVER['HTTP_X_FORWARDED_FOR']) || isset($_SERVER['X-REAL-IP'])) {
    echo('<font style="color:grey">Transparent Proxy</font>');
} elseif (isset($_SERVER['HTTP_VIA'])) {
    echo('<font style="color:blue">Anonymous Proxy</font>');
} else if (!isset($_SERVER['HTTP_VIA']) && !isset($_SERVER['HTTP_X_FORWARDED_FOR']) && !isset($_SERVER['HTTP_X_PROXY_ID'])) {
    echo('<font style="color:green">Elite Proxy / No Proxy</font>');
} else {
    echo('<font style="color:blue">High Anonymous Proxy</font>');
}

echo "<br> <br> <b> Joe's custom prints: </b> <br>";

/* Joe's Code */
if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])){
	echo "HTTP_X_FORWARDED_FOR exists and is: ".$_SERVER['HTTP_X_FORWARDED_FOR']."<br>";
}
if (isset($_SERVER['X-REAL-IP'])){
	echo "X-REAL-IP exists and is: ".$_SERVER['X-REAL-IP']."<br>";
}
if (isset($_SERVER['HTTP_VIA'])){
	echo "HTTP_VIA exists and is: ".$_SERVER['HTTP_VIA']."<br>";
}
if (isset($_SERVER['HTTP_X_PROXY_ID'])){
	echo "HTTP_X_PROXY_ID exists and is: ".$_SERVER['HTTP_X_PROXY_ID']."<br>";
}