The only way to be absolutely certain is to say something illogical, like "This statement is false" or "You only hurt the ones you love" or "What can brown do for you?" If their head doesn't explode into a shower of electrical sparks, you are dealing with a real person.
The standard, tried-and tried-and method of talking to a real human person at the post office would be to actually go to a post office --either a main post office or a branch post office -- and actually walk in during their posted hours and wait your turn on queue and speak with a postal clerk at the counter. This tends to yield good results.
My experience is thato you must go to the post office to talk to a living body if the automative phone service doesn't answer your question or help you.
Apparently you have to walk into the building as no option seems to get you a human on the phone system.
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