Historically first time the effect of attraction of small objects for a static charged object was discovered for amber rubbed with textile material like wool. This discovery (better said observation) was made by ancient Greeks more than 3000 years ago, and thus the static charge built was named "electron" which is Greek for amber. Thus by DEFINITION the static charge that builds on amber rubbed with wool (textile material) is negative. To know if an object is positively or negatively charged it is enough to bring it near a piece of rubbed amber. If the two objects attract than the object is positively charged, if the two objects repel then the object is negatively charged.