Signal person is one of the few roles in construction where the standard spells out both when the role must exist and what the person has to prove.
When one is required. A signal person must be provided in three situations: the point of operation — the load travel or the area at or near load placement — is not in full view of the operator; the equipment is traveling and the view in the direction of travel is obstructed; or, because of site-specific safety concerns, either the operator or the person handling the load determines one is necessary (29 CFR 1926.1419(a)). The third trigger matters: a rigger who says "I want a signal person for this pick" has invoked the standard.
How qualification works. The employer of the signal person must ensure the person meets the qualification requirements before giving any signals, using one of two options: documentation from a third-party qualified evaluator, or an assessment by the employer's own qualified evaluator who documents the determination (29 CFR 1926.1428(a)). The qualification requirements are to know and understand the type of signals used — including the Standard Method for hand signals — to be competent in applying them, to have a basic understanding of equipment operation and limitations including crane dynamics in swinging and stopping loads and boom deflection, and to know the relevant requirements of 1926.1419 through 1926.1422 and 1926.1428 (29 CFR 1926.1428(c)(1)–(4)). All of that must be demonstrated through an oral or written test and a practical test (29 CFR 1926.1428(c)(5)).
Losing the qualification. If a signal person's subsequent actions show the person does not meet the requirements, the employer may not allow them to continue as a signal person until retraining and a fresh assessment confirm they do (29 CFR 1926.1428(b)).
A hard hat with "SIGNAL" on it is not the qualification. The documentation is, and it should be reachable on the site.