To any reader who still doesn't knew anything about tungsten; tungsten is one element that has just about the same density as gold. Alloying gold with tungsten would not work for several reasons but a coin with a tungsten center and gold all around it could not be detected as counterfeit by density measurement alone. Most of bank only check the density of the gold when they receive it from seller. Its density, similar to that of gold, allows tungsten to be used in jewelry as an alternative to gold or platinum. Tungsten's resistance to high temperatures and its strength in alloys made it an important raw material for the weaponry industry.
The name "tungsten" (from the Nordic tung sten, meaning "heavy stone") is used in English, French, and many other languages as the name of the element. Tungsten was the old Swedish name for the mineral scheelite. The other name "wolfram" (or "volfram"), used for example in most European (especially Germanic and Slavic) languages, is derived from the mineral wolframite, and this is also the origin of its chemical symbol, W. The name "wolframite" is derived from German "wolf rahm" ("wolf soot" or "wolf cream"), the name given to tungsten by Johan Gottschalk Wallerius in 1747.
I check if there is any way to test for tungsten gold. Some suggested to check the gold electric conductivity with Ohmmeter. An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Tungsten is way way harder than gold so another test is by pushing a pointy steel thing into the coin. If the coin is just gold plated tungsten then the point would penetrate a very short distance and make nothing more than a tiny pin point dent. If the coin was actually gold you would be able to make a large dent in the surface. An automatic center punch would work well I would think. The only problem is that all your coins would then have a noticeable dent in them. If you only care about bullion value then I suppose the dent wouldn't matter, but for a collector this test would be unacceptable.
Source: Wikipedia, Kitco.com