You could pass on a dynamically (server-side) generated and logged token from site A to site B (for instance, as a GET parameter), then you can check the tokens later to see if they match. Or better yet, use some kind of inter-server mechanism (like memcached) to check the token when the request reaches site B.

Without some dynamic serverside control of server A, you won't be able to get even remotely reliable prevention of spoofing.

updated for clarification

"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re: How to reliably get referrer? by Joost
in thread How to reliably get referrer? by punch_card_don

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