How do scientist measure/know the half life of isotopes?

i have been trying to find out how scientists know the age of things especially earth. i understand from my investigations that uranium in rocks hold isotope crystals the most common isotope being U 238 and by its decay they can calculate that it is 4.46 billion years. but how do they measure the decay of uranium isotope u238 to know its exact date of 4.46 billion yrs????