Source

File: wp-includes/rss.php

function check_cache ( $url ) {
		$this->ERROR = "";
		$cache_option = 'rss_' . $this->file_name( $url );

		if ( get_transient($cache_option) ) {
			// object exists and is current
				return 'HIT';
		} else {
			// object does not exist
			return 'MISS';
		}
	}