If there is a body whose charge is known, then the unknown charge of another body can be determined by observing the interaction with a body whose charge is known. You can use an electrometer whose charge is known. If, when the body touches the electrometer, the charge of the latter increased, then the charge of the brow is the same as that of the electrometer, if it decreases, then the opposite.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.