A useful function written in PHP to check whether a website has a backlink to yours and if it is a Google Friendly one.

backlinkchecker.php

<?php

/*
 * $backlink - the link we are looking for
 * $url - the website against we are checking the $backlink
 *
 */

function backlinkCheck($backlink, $url) {

	$backlinkexists = false;
	$backlinknofollow = false;

	$ch = curl_init($url);
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
	curl_setopt($ch, CURLOPT_BINARYTRANSFER, true);
	$content = curl_exec($ch);
	curl_close($ch);

	$dom = new DomDocument();
	libxml_use_internal_errors(true);
	$dom -> loadHTML($content);
	$anchors = $dom -> getElementsByTagName('a');
	libxml_clear_errors();

	foreach ($anchors as $anchor) {
		$link = $anchor -> getAttribute("href");

		//check if the link exists among links on the $url
		if ($link == $backlink) {
			$backlinkexists = true;

			$anchorhtml = strtolower($anchor -> C14N());

			if (strpos($anchorhtml, 'nofollow') !== false) {
				$backlinknofollow = true;
			}

			break;
		}
	}

	if ($backlinkexists && !$backlinknofollow) {
		// backlink exists and it doesn't has a rel attribute with nofollow value
	} else if ($backlinkexists && $backlinknofollow) {
		// backlink exists and it does has a rel attribute with nofollow value
	} else {
		// backlink doesn't exists
	}

}

?>

If you find it useful, use it.