Take on the proof of being unvaccinated

Some dive in southern California is requiring that all patrons be unvaccinated and saying they will need to show proof of the status in order to get served the meatloaf. This brings up the obvious question is how you prove that you are no vaccinated, will the restaurant scour your wallet for a vaccination card? Do they look in some giant government database to cross check your claim? Do they breath Covid in your mouth to see if you get sick??

I think the only proof of somebody being unvaccinated at this point would be if you see them wearing a red baseball cap.