I wouldn't say that it's easy. You at least need some tungsten, uranium-238, rhenium, platinum, iridium, osmium, or chengdeite (Ir3Fe) most of which are nearly as expensive to obtain as actual gold. Any other material that is denser is probably also radioactive, and only available from nuclear fission reactors.
Os 22.59 kg/L $13k/kg $294k/L
Ir 22.56 kg/L $22k/kg $496k/L
Pt 21.45 kg/L $29k/kg $622k/L
Re 21.02 kg/L $69k/kg $1450k/L
Au 19.30 kg/L $40k/kg $772k/L
W 19.25 kg/L $50/kg $960/L
U 19.1 kg/L $150/kg $2900/L
The obvious choice for counterfeiters would be tungsten. A gold bar that has been hollowed out and filled it with tungsten is detectable with ultrasound. Counterfeit tungsten coins with known dimensions can be detected by measuring electrical resistance, or by doing a "ring" test, which is striking the coin with a hard object and listening to the sound it makes. The difference between the sound of a gold coin and a tungsten fake is clearer than the difference between a genuine B20 bell-bronze crash cymbal and a cymbal made from cheap brass. The gold coin will ring, while the tungsten coin will barely sustain a dull thunk.
The other noble metals are available in extremely limited supply, and usually comparable in value to gold or platinum anyway.
Devices that test for the density of gold coins are already cheap and widely available. Using a Fisch device and dropping the coin onto a countertop is just as easy as using the counterfeit detector marker, checking the watermark and color-shift inks, and looking at the security strip under a UV light.
