The main symptom of transient global amnesia is being unable to create new memories and remember the recent past. Once that symptom is confirmed, ruling out other possible causes of amnesia is important. You must have these signs and symptoms to be diagnosed with transient global amnesia: 
Sudden onset of confusion that includes memory loss, seen by a witness
Being awake and alert and knowing who you are, despite memory loss
Normal cognition, such as the ability to recognize and name familiar objects and follow simple directions
No signs of damage to a particular area of the brain, such as being unable to move an arm or leg, movements you can't control, or problems understanding words
 More symptoms and history that may help diagnose transient global amnesia: 
Symptoms lasting no more than 24 hours and generally shorter
Gradual return of memory
No recent head injury
No signs of seizures during the period of amnesia
No history of active epilepsy
 Another common sign of transient global amnesia due to the inability to create new memories includes repetitive questioning, usually of the same question — for example, "What am I doing here?" or "How did we get here?"