The latest high-tech equipment permits reliable results to be obtained even with microscopic samples.
What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.
(see link) Scenario: I heard about a group of people (yes they happen to be creationists with an agenda, but this should be irrelevant to the question I am posing!
) who obtained some samples of rocks from a lava flow from Mt Ngauruhoe in New Zealand.
They claim that the rocks they obtained were from a lava flow which came out of the volcano in 1945.
They sent these rocks to 2 labs and had them dated by potassium-argon dating to be between 270, 000 and 1 million years old.