If you can actually transmute copper \(Cu\) into gold \(Au\), some of the gold will be transmuted into Roentgenium \(Rg\) which is radioactive. Transmuted gold can then be detected by the presence of \(Cu\), \(Ag\) and \(Rg\) isotopes impurities; and that it is radioactive.
Refining transmuted gold will add costs and raise the minimum price of such gold. So we do not have a runaway economic meltdown as yet.
