Simply counting the number of rings will give one a fairly good idea of the age of the tree.
Periods of heavy rain and lots of sunshine will make larger gaps of growth in the rings, while periods of drought might make it difficult to count individual rings. When a given quantity of an isotope is created (in a supernovae, for example), after the half-life has expired, 50% of the parent isotope will have decomposed into daughter isotopes.
Increasingly, the other techniques are used, such that probably no one technique dominates for meteorite dating.
Rb-Sr is a good example for explaining the process, however.
From his research, our evolutionary geologist may have discovered that other geologists believe that Sedimentary Rocks A are 200 million years old and Sedimentary Rocks B are 30 million years old.
Thus, he already ‘knows’ that the igneous dyke must be younger than 200 million years and older than 30 million years.
Only the latter two "extinct" nuclides are used in dating.
The geologist may have found some fossils in Sedimentary Rocks A and discovered that they are similar to fossils found in some other rocks in the region.