Referrer

The referrer header tells the receiving server which URL sent the click. For affiliate programs it is the cheapest traffic-verification tool available: claimed “SEO review traffic” arriving with adult-site or URL-shortener referrers is a conversation waiting to happen.

Its limits

Referrers are trivially spoofed or stripped (HTTPS→HTTP transitions, privacy tools, in-app browsers), so absence proves nothing. Treat referrer data as one input to traffic-quality review alongside SubIDs, conversion behavior and declared sources — a lie-detector, not a ledger.