Presuming you can create an alloy with the same density as gold, I imagined you could also test it's conductivity. I think performing both tests would be enough.
The only metals with which you could make an alloy with the same density as gold, but cheaper than gold, are uranium and tungsten.
Other metals would require too big additions of expensive rhenium/osmium/iridium/platinum to match the density of gold.
The best choice for matching the density of gold is tungsten, but even with that the cost for an exact match of the density would be high. The tungsten objects that are found easily in commerce have a density significantly lower than gold, because they are made from tungsten powder sintered with nickel, not from pure tungsten, which is hard to melt.
The conductivity test is good, but not easy to perform when the object has a complex form. Surface conductivity is easy to measure on any object, but the object could be plated with pure gold, so surface conductivity would show no difference.
For a gold bar of standard dimensions, it should be easy enough to make a text fixture allowing the measurement of the bulk conductivity.
It is rather expensive to make objects of pure tungsten. The tungsten objects that you see for sale are not pure and they have a density more than 5% lower than gold, which is easy to detect by weighing.
