Robert Millikan's
oil-drop experiment to measure the electronic charge (since repeated by generations of physics students) measured the force on tiny charged droplets of oil suspended against gravity between two metal electrodes. Knowing the electric field the charge on the droplet could be determined. Repeating the experiment for many droplets, this was found always to be one or a few units of a common value. This was taken to be the charge on a single electron.
A version of the oil drop experment has subsequently been used to search for free quarks (which, if they exist, would have a charge of 1/3 e), without success. Current theories of quarks predict that they are tightly bound and will not exist in a free form.