By convention when we rub a glass rod with the Silk cloth it acquires positive charge and when we rub ebonite rod with her or wool it requires negative charge similarly if we rub the balloon with Silk cloth it will also gain positively charged to know whether an object has a positive or a negative charge what we can do is we can inflate a balloon and rabbit with the Silk cloth and as we know that by convention the charge that will be acquired by the balloon will be positive we can put we can keep any body charged body near it and if attraction will be there it means that the first one the balloon was rubbed by Silk cloth and it acquires positive charge and now there is attraction between the balloon and the charged body you have kept near it means that the charged body have negative charge on it because as we have read that like charges repel each other and unlike charges attract each other so we can say that the object be kept near the positively charged balloon was having negative charge and if it would have shown repulsion it means that the object was having positive charge because like charges repel each other

It's very simple to understand in this way... The one who gives get's positive charge and the one who receives gets negative charge