This is done through a number of fiendishly complicated techniques that usually involve calculating how much of a specific chemical (such as carbon) is present in the fossil, and then matching that to the known and constant rate of decay of that chemical.
Leswi swi endliwa hi nhlayo ya xinghana kambe leyitikaka leyitalaka kuva yi katsa ku hlayela leswaku kuna khemikali ya muxaka wihi (kufana na carbon) leyi kumekaka eka ti fossil, kutani swipimanyisiwa na nhlayo leyitivekaka yak u bola ka khemikhali yoleyo.