I have done something somewhat similar to sauoq's suggestion: add 2 parameters to the url (e.g. gwhite.com?referrer=traveler.com?id=0949), then validate the "id" parameter with a request back to the referrer. The request needs to timeout on the "referrer". (I use an MD5 value for the id, and validation is a bit more complex, but the idea is similar.) If you are not too concerned about security, you could make the id be some function (e.g. MD5) of the referrer, perhaps the destination page, and a changing key such that it could be validated directly on your machine.

HTH, --traveler