By experimentation, scientists have determined which rocks are suitable for various dating techniques. For K-Ar, for example, igneous rocks are good candidates for testing because they formed directly from molten magma and have a simple history. Sedimentary rocks do not work because they are made up of a mixture of deposits of many other types of rocks, each of which would point to a different age. At any rate, scientists have devoted a great deal of effort to determining exactly which dating methods are appropriate for which types of rocks. Creationists want the world to think that geologists just grab a rock and throw any old radiometric test at it and poof - there's the age of the rock.
Reality is far more complex.
If you examine the extensive research in the field of geochronology, you will see that one of the most important criteria in dating a sample lies in choosing an appropriate dating method for the sample. A great deal of effort goes into the sample selection, and the choices are made before the analysis, not on the basis of the results. Mistakes are sometimes made but are usually caught by the various checks employed in the well-designed experiment. The most compelling argument for an age of the earth of 4.5 billion years are the large number of independent tests that have been used to confirm this date. These tests have been performed on what are thought to be the earth's oldest surviving rocks, meteorites, and moon rocks.