The blood sugar level will give a feedback to certain cells of pancreatic islets via receptor which those cells will respond by producing responsible hormone to regulate the blood sugar level in normal condition.
From what i know, there are 2 ways that the pancreas is aware of glucose blood level. One is the humoral one, when by the amount of glucose that enters the cell, it can detect variations, and another is the nervous one: when the chemoreceptors in blood vessels detect variations in glucose blood level, the send the info to the hipothalamus. By the vegetative impulses, it commands the secretion of either insulin or glucagon. Parasimpathetic stimulates insulin production, sympathetic stimulates glucagon secretion