How come we can determine the glass rod will have positive charge and the plastic one will have negative charge and not vice versa? Is there any reason to determine it or is it just like that?

Dear student, Rubbing both the rods on different materials will induce different charges . It is not necessary that either glass rod or plastic rod always aquires negative charge or positive charge. It depends upon the material on which it has been rubbed. Some materials are electron friendly while some looses electron depending on their reactivity. So, it can be found by experiments. Hope it helps you. Regards