Diodorus Siculus (ca 60-30 BCE) ¶ 31 '... They [the Gauls] also make use of seers, who are greatly respected. These seers, having great authority, use auguries and sacrifices to foretell the future. When seeking knowledge of great importance, they use a strange and unbelievable method: they choose a person for death and stab him or her in the chest above the diaphragm. By the convulsion of the victim's limbs and spurting of blood, they foretell the future, trusting in this ancient method. They do not sacrifice or ask favors from the Gods without a Druid present, as they believe sacriface should be made only by those supposedly skilled in divine communication."
Strabo (63 BCE to 21 CE at least)
¶ 4.4.4 "As a rule, among all the Gallis peoples three sets of men are honored above all others: The Bards, the Vate, and the Druids. The Bards are singers and poets, the Vates overseers of sacred rites and philosophers of nature and the Druids, besides being natural philosophers, practice moral philosophy as well...
¶ 4.4.6 "... The Romans put a stop to these both these customs [taking the heads of slain enemies] and to the ones connected with sacrifice and divinition, as they were in conflict with out ways: for example they would strike a man who had been consecreted for sacriface in the back with a sword, and make prophecies based on his death spasms; and they would not sacriface without the presence of Druids..."