Here’s a concise review of — what it is, how it works, its reliability, and its limitations. 🔍 What Is an “iCloud Check by Serial Number”? It’s a service (often found on third-party websites or paid tools) that claims to tell you whether an Apple device (iPhone, iPad, Mac, etc.) is still linked to an iCloud account (i.e., has Activation Lock enabled) — using only the device’s serial number .