Why is it more difficult to determine the electric charge on an object than the mass of an object?

Answer 1

No matter what one is measuring, there must be a standard against which to measure it, and a means of comparing it to the standard. In the case of measuring charge, the object being measured must be compared to a standard charge by some means such as measuring their mutual attraction or repulsion, moving it at a known velocity while the resulting magnetic field is compared to a standard, or some other means. Measuring the brightness of a light, a time interval, or anything else requires some such operation.

For mass, there is a particularly easy way to make measurements because the Earth's gravity provides a very accessible and consistent reference. All that is needed is a standard mass and a balance. So measuring an object's mass is usually easier than most of its properties. Lengths can also be rather easy.