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

Here's a gadget that would work -- mailbox chime:
http://www.improvementscatalog.com/product.asp?product=276342zz&dept%5Fid=12300&subde pt%5Fid=12330