Nominee arrangements are lawful in many jurisdictions and are a normal product of the corporate services industry. The concealment arises when the nominee is the only name a registry, a bank or a counterparty ever sees.
The tell is usually workload. A single individual appearing as director of dozens or hundreds of unrelated companies across unrelated sectors cannot be directing any of them, and that pattern is visible in any bulk company register. The corresponding document is the side letter or declaration of trust in which the nominee acknowledges acting on another’s instruction, which is what investigators look for once they have a search warrant or a leaked archive.