For an e-commerce site, deciding if someone is a returning customer starts with checking identifiers like cookies, logged-in account IDs, or device fingerprints. It also compares purchase history, saved shipping addresses, and payment tokens to see if previous transactions match the current session. Behavioral signals such as browsing patterns, cart recovery attempts, and login frequency help confirm repeat activity across visits. Systems may also check IP address consistency, device type, and session tokens stored in browsers or apps. Email reuse, loyalty program IDs, and linked payment methods further strengthen identification across platforms and devices.