To answer your question directly: a lab could melt a sample and collect the gases that were released. Some of that gas would be argon, most likely. Feeding the gas to a mass spectrometer wouldshow that argon to be the isotopes -36, -38, and -40, all non-radioactive. Any of the other (radioactive) isotopes of argon have short enough half-lives that they would be completely decayed after sitting in a rock for just the 6000 years that Faith would give you since the world formed.
One radioactive inert gas you can find in rocks is radon. If there is uranium in the rock, it will decay (very slowly) and continuously supply traces of radon - though the radon itself then decays, finally ending up as lead.
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