Detectable if you measure density carefully enough (though do people? I don't know). If you did that with plutonium instead (way more expensive than gold obviously, so it would be a silly thing to do) then you could get it as close as you wanted.
As an interesting sidebar, there's a series of photos of some of the scientists on Tinian posing with a small box containing the core of the Fat Man bomb - a 3.5" diameter sphere of Plutonium that was apparently worth at the time the equivalent of about $5 billion dollars.
It's certainly detectable with precise instruments, but it's not practical to test each and every bit of gold one acquires so thoroughly. Hence the opportunity for counterfeit gold that passes casual inspection and weighs about the right amount.
