Most people think scientists just look at a fossil and guess its age — actually, there are multiple techniques with wildly different accuracy ranges, and scientists cross-check them. The key insight: different dating methods work for different materials and timescales. Radioactive decay is the foundation, but it is not the only tool.
Atoms of certain elements are unstable and decay into other elements at a predictable rate. This rate is unaffected by temperature, pressure, or chemical reactions — it is one of nature's most reliable clocks. Scientists measure how much of the original element remains vs how much has decayed, and calculate backwards to find the age.
You do not always need absolute ages. The principle of superposition says: in undisturbed rock layers, older layers are underneath younger layers. So if you find a fossil in layer 5, it is older than anything in layers 1-4 above it. This does not give you a number, but it orders events. Fossils within the same layer are the same age, and you can match layers between sites using distinctive fossils (index fossils) that only existed during specific time periods.
If a fossil layer contains volcanic ash, scientists can date the ash using potassium-argon dating, pinpointing the fossil age. If bones are found with artifacts of known age, carbon-14 corroborates. The reason paleontologists are so confident about dinosaur timelines is not one measurement — it is hundreds of concordant measurements across continents using multiple methods. When every method agrees, you have high confidence.
Pro tip: The biggest misconception is that scientists date the fossil itself. You usually can not — fossils are dead organelles (bone is no longer metabolizing). Instead, scientists date the surrounding rock (using radiometric methods) or the layer context (using relative methods). The fossil's age is inferred from its geological position. This is why volcanic ash layers near fossils are so valuable — they can be dated directly.