Use different methods and see if they give the same answer. Cross check those answers against other data, such as the known rate of geological and biological processes such as gene-clocks. Further cross-check assumptions such as the constant rate of atomic decay and constant lightspeed by observing atomic decay rates in distant supernovae. Numerous different methods of calculating this stuff come up with the same answers (within reasonable experimental error) every time.
Further cross-check assumptions such as the constant rate of atomic decay and constant lightspeed by observing atomic decay rates in distant supernovae.