Anesthesiologists rely on indirect signs, blood pressure changes, and increased heart rate, because they assume that if you become awake during surgery you will experience pain , fear and anxiety which will raise your blood pressure and make your heart beat faster. But not everyone will experience higher blood pressure and an increased heart rate. Their condition is often life threatening and heavy anesthetic could kill them. Other people who are at greater risk, also because heavy anesthetic could harm them, are the elderly, children and women giving birth by cesarean section. But it can happen to others.
Some surgeons are skeptical.
Certain anesthetic agents induce specific dream-like states, so you wonder if the patient is actually having a dream or whether they were actually awake during the procedure. But most surgeons acknowledge that it does happen and it can have a profound effect on a patient. It could lead to post-traumatic stress reaction. There are a handful of researchers around the world who are working on a device to alert anesthetists that a patient is waking up. One area being looked into is sound.