Hollow gold bar A scammer got a hollow gold bar and fills it with a combination of lead and air, with the same average density as gold. 
What's the simplest way of discovering the fraud? I know that x-rays will see it, but are there simple means for analyzing it? (without destroying it)
 A: Edit: I realize now that measuring the difference in resistance is not practically easy as the resistivity difference isn't significant, and there are numerous other measurement challenges including surface areas for contacts creating a significant enough difference for this method to work. Other methods may be more suitable.
If it's a cuboid, then I think a simpler way would be to measure its resistance.


*

*Use Suitable contacts

*Measure resistance

*Use R=ρL/A and find out resistivity ρ.

*Either use the table found here and check if it matches up


You can also use similar method for other properties like 
-Thermal conductivity: Heat one end and monitor how quickly the other end's temperature rises, etc and compare to expected value or just another gold bar of similar dimension(which shouldn't be too difficult)
-Ultrasonic pulse (like'Pulsar' mentioned)
-A similar technique can be used with other rf wave reflection analysis. The pattern reflected would differ from a solid gold bar.
-Another method that comes to mind is Electrodynamic suspension. You can suspend it in air, but the eddy current will be different because it is hollow, and thus the induced magnetic field strength will be different. For a given time-varying magnetic field function, the response would differ.
But I personally think that the resistance measurement is the easiest.
All these are non destructive, btw. (Not so much for the scammer.)
A: This answer will not make me popular because it gets people up to speed fast on protecting themselves from thieves and levels the playing field for people who like to maintain their advantage over others.
There are a few methods we use to determine if gold is bunk or real...Methods that test if your gold is hollow, filled, alloyed (and the alloy percentages and what the alloy material might be) and if it is plated bait material for duping people unaware of practical testing methods (Tungsten has been known to bait gold bars (historically)).


*

*You can cut the bar in half...You will then know immediately of you
got bunk gold.

*You can do a specific gravity check of your gold.  There are scales
designed for this purpose and use weight by displacing water.  You
have heard that gold is roughly 19 times as heavy as water.

*Next you can do a spectral analysis using XRF Spectroscopy.  This is a newer method for testing requiring you to drill a small
core sample and zap it using a handheld gun or placing it in a table
top unit.  This is very accurate.

*You can do scratch tests which are old-school comparisons using a a
ring of gold and silver karat keys and acid to do this side-by-side
comparison.

*Another method is acid digestion/precipitation using aqua regia
super acid (Nitric/Hydrochloric acid mixture). This is a more common
method and this fundamental method is part of a foundation called
qualitative/quantitative measurement and was taught to high school
kids in the 40's and 50's. To precipitate you would use a mixture of
some sodium and sulfur combination or by controlling the PH levels
via heat and sodium hydroxide gold drops out of solution when you raise the PH
level between 9 and 10. If you go higher on the scale you start to drop
other unwanted metals. This is usually done with smaller amounts of gold

*If you don't trust yourself you can send a sample of your material
to ALS Global.  This is the one of several Canadian companies
that are trusted for materials analysis (based on a USGS study I
found 5 years back).  The turn around time on analysis is usually
2-3 days.  You don't get your samples back, and time can drag
depending on the quantity of materials you are testing. They have
packages to select from and they are reasonably priced (if you are
not in the know on testing or just rather have the experts deal with
it then use ALS). I have also done this with very small samples.
In the gold and silver markets there are all types of things you have to be careful about. The best method is to vet your sources.
A: Send an ultrasonic pulse through the gold bar and analyse the returning wave. This technique is actually used to detect impurities in gold bars.
To quote this article:

Where the wave encounters a region of material with different physical properties – particularly the density and elastic constants – to the rest of the metal, the beam is affected in a number of ways:
Scattering (attenuation) can occur as a portion of the wave is reflected or reradiated from small regions of different material, or from an irregular interface between two larger regions of different material. This manifests itself as a reduction in the amplitude of the wave
A change of arrival time of the wave at the detecting transducer can occur if the wave passes through a significant region of material where the ultrasonic velocity is different
Refraction/reflection of the wave – or where there is a relatively (compared with the wavelengths of ultrasound used) smooth interface between two regions of different material, a portion of the wave will be reflected from this interface. Impurities will result in the wave being refracted at a different angle

