So aside from drilling your coins, is there any easy way to figure if they are gold or not? I was thinking I heard of people bouncing the coins and listening, but that was from a book (maybe Job: A Comedy of Justice - Heinlein).
If they are eagles...weigh and measure them. (Like the little teeter-totter thing)
A gold eagle is a gold/silver/copper alloy with a density that lies in a really weird place (substantially denser than lead, mercury, and other things, but less than the dense refractories/platinum group metals.
Meaning yes you -could- core one, but it would be really tough to create an approximate alloy with lower cost materials yet the same density.
It also wouldn't be worth it.
I like the bending idea, for bars.
There is an even easier solution. Just hit it with a neutron source and do a gamma spectrogram. Anything other than gold would show up like a turd in a rice bowl.