Hi RedOne, I'll answer this with reference to the K-Ar method, as it is the one I'm most familiar with. This is a very basic answer, do go and look at scientific references to see a more complete story.
We regularly use Biotite crystals to date the rocks, as biotite is a very common mineral formed when a rock crystallises from magmas.
The chemical formula of biotite is K(Mg,Fe)3AlSi3O10(F,OH)2.
Because Ar is a noble gas, it doesn't take part in any chemical reactions and can thus not be incorporated in the crystal structure of Biotite when the crystal forms. There would thus be no Ar present in the crystal structure of biotite when the crystal is formed. When we measure the Ar in the crystal structure of Biotite, we know that all the Ar present in the crystal structure is a result of radioactive decay of K (Ar can be incorporated in the form of fluid or air bubbles in the crystal, but these are not part of the crystal structure and we can eliminate these as far as possible).
As long as the crystal exists, we can thus measure the K and Ar ratio in the crystal structure. Furthermore, we can test the chemical formula of that crystal to test the chemical composition of that biotite crystal to see if the chemical formula is different; we can look at it under the microscope to see if we can find phenonema like zoning, etc. to see if the rock has been partially melted since formation and other elements could have been incorporated as part of the crystal, etc.
Geologists are not as dum as creationists try to convince people the are!
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