Age of fossils in the past was generally determined by radio-carbon method and other methods involving radioactive elements found in the rocks. More precise methods, which were used recently and led to the revision of the evolutionary periods for different groups of organisms includes -

Radiometric dating (carbon-14, potassium-argon, uranium-lead) measures the decay of radioactive isotopes in fossils or rocks.
Electron spin resonance (ESR) dating measures the number of trapped electrons that build up in materials such as tooth enamel and shells due to natural radiation over time. It can date fossils that are too old for radiocarbon dating and is more precise for such material.
Fossil DNA (ancient DNA) can be extracted and compared with DNA of living organisms. The differences, combined with the molecular clock idea, give independent estimates of when groups separated.
These methods have led to revision of the time periods of evolution of several groups, including humans.
So the more precise recent methods are electron spin resonance (ESR) and fossil DNA.