quello che cerchi e' il document.referrer
troverai esempi cercando su google

lato server hai alternative piu' affidabili,
per es. in php hai $_SERVER['HTTP_REFERER']

...e se ti stessi chiedendo che fine fa la seconda r...
Referer is a common misspelling of the word referrer. It is so common, in fact, that it made it into the official specification of HTTP the communication protocol of the World Wide Web and has therefore become the standard industry spelling when discussing HTTP referers.


ciao