Its hard to imagine that a bureaucracy like the USPS doesn't have a
whole book of rules concerning this.

"Rural" mailboxes in front of individual homes (or clustered somewhere
on a block of individual homes, as I have seen in California) is a
relatively recent development. Before the suburbs sprouted, farmers
would have a cluster of mailboxes at intersections, so your own box
might be miles from your home. At that time, the postman would raise
the red flag when he put mail in your box, so you could see you had mail
from your tractor or truck, and make the trek to get it. I don't recall
any device to show that there was mail to be picked up by the postman,
but he didn't really need one as he would stop by the cluster of boxes
to make his deliveries. Of course, at that time there was not the flood
of advertisements mailed that there is today.

Perhaps the growth of suburbs and advertising mailings has changed this,
but certainly the post office has a set of rules covering this; I know
they had some regarding the size of mail boxes a few years ago.

Kurt Gavin wrote:
I'm talking about those standard roadside USPS boxes.

Mine's in front of the house and I'm wondering if there's a gadget that can
tell you by sight, or maybe electric trip switch, if there's something in
the box.

Thanks