In order to know the charge sitting on an object, you need another object with a known positive or negative charge. If we rub a piece of glass with silk, it will have a positive charge. If you rub a piece of amber with fur, it will have a negative charge. Use whatever you have. Consider, a piece of glass rubbed with silk. If the object is repelled by the glass, then it has the same charge as the glass (positive). Otherwise, that object has a negative charge.