There are many methods and techniques for this, from photometry measurement to ionized mass spectroscopy.  One of the most accurate and speedier methods for quantifying a substance is liquid scintillation. 

The use of radiation counters has been prevalent in the scientific community, and the Geiger-Muller tube has been seen in use around nuclear power stations and even used in the study of geological activity, such as volcanoes.  The principle behind the Geiger counter is much the same as a liquid scintillation counting device, except it is not nearly as accurate, and can usually only determine the presence of radioactive isotopes.

The first item on the list of things to accomplish prior to testing is making up a scintillation "cocktail".  This mixture can be radioactive water, such as tritium, and a solvent like benzene.  Fluors, or the particles that will glow during the radioactive decay of the water, are also necessary to emit light that the counter will detect. The cocktail must be meticulously made up, due to possibilities of contamination and erroneous results.

Using radioactive water, the scintillation device can measure amounts so small that individually radioactive labeled molecules and atoms will activate the Fluors in the scintillation cocktail.  This comes about by the release of a Beta particle of gamma radiation.  The particle charges away from its bonded molecule and strikes the Fluor, which emits a photon of light.

The counting device can detect those tiny increments of light using a photomultiplier, a sensitive device that converts photons into an electrical signal.  This signal travels to the counters on board computer, which in turn displays the amount detected.