Read this topic, covers this subject pretty well
http://forums.silverstackers.com/to...coins-with-an-ultrasonic-thickness-gauge.html
What it boils down to is: Buy poured bars probably bigger than 5 ounce and the $13 scales off ebay plus the $150 ultrasonic thickness gauge from ebay and that will have you covered.
If you want the story on how to use the thickness gauge, go here
http://about.ag/UltrasonicThicknessGauge.htm
If you buy tiny poured bars that are all stamp and no bar, minted bars without a flat surface or coins then you will need a $20,000 XRF machine which still won't tell you what is in the middle of the metal in question.
I've read some really sh*t pages on testing specific gravity that are just plain wrong or overly convoluted.
This is the "right" way to do and think about it.
1) weigh your metal dry (you do have your quality $13 set of scales don't you?).
2) put a small plastic container of water on your scales (careful not to overload it) Tare it so it reads zero.
3) hang the metal in the water, not touching the sides or the bottom. Do this using 0.1mm nylon monofilament available from a sewing shop. (tip: support the weight over some fixed object, so the reading on the scales stops jumping all over the place) You are measuring the mass of displaced water here.
4) Take the number you got in step 1 and divide it by the number you get in step 3. You should get something pretty close to 10.49 depending on how thick your monofilament is and how accurate your scales are.
This result can be faked by silver plating the right lead alloy, but then the Ultrasound test will pick up on that act of deceit
