We demonstrate a method for determining the charge state of surface arsenic vacancies on p-type GaAs(110) by using scanning tunneling microscopy. The method is based on the compensation between hole depletion at isolated arsenic vacancies and hole accumulation at ionized dopant atoms whose charge states are known a priori. Detailed analysis shows a one-to-one compensation between the charged vacancies and the ionized dopants, indicating the isolated arsenic vacancy charge of + 1. This method can be extended to determine quantitative charge states of other point defects (positive or negative).