Could one have some kind of "registry of approved visitors"? You can have lifelong friends you would trust as much as a spouse, or family members you don't trust. For a hospital clerk to verify that someone is a family member (or a spouse), if the last name on their physical ID doesn't match, would require some kind of database lookup in any case.

It'd probably get complex. Like is person X allowed to visit you, person Y is allowed to approve certain non-invasive procedures for you, and person Z is allowed to make life-altering decisions.

As a paramedic, if you're conscious you'll typically get asked upon admission, "If anyone calls are we allowed to tell them you're here, and do you have any specific desires about visitors?".

If you're unconscious or unable to elucidate these things (which often correlate to the more serious end of the spectrum), not so much.

When I was hospitalized a long time ago we found that my partner was unable to see me and practically unable to act as my proxy (because they wouldn't let her into the hospital beyond the front office), even with a power of attorney (and her being my attorney, too).

Her telling them that she was my husband instantly resolved the problem, no questions asked. It's not like they have any immediate way to check.