QUOTE(Coinmelt @ Mar 25 2008, 10:13 PM)

I've tried the tissue test already.
Are you sure you have at least one of each composition?
Edited to add: The only non-destructive sure-fire method that I know of would be to test the specific gravity of your coins.
The specific gravity of silver is 10.5, while Copper and nickel are both <9, so the overall SG of a .900 silver coin (~10.35) will be greater than that of a .500 silver piece (~9.75).
An internet search should turn up several ways to measure relative specific gravity without fancy instruments.