Can an ECG show if there has been a previous heart attack?

My mother passed away on Friday night after suffering a heart attack. The cause on the death certificate is: Anterior ST Elevation Myocardial Infarction. She suspected that she had angina as she had begun to suffer pain on exertion in the past 3 months and was due to see a cardiologist for it in the near future. In her last couple of days the pain became worse and she booked an appointment with her General Practitioner for 5pm on Friday. He listened to her heart and took her blood pressure - he said that she should take it easy and go to the forthcoming appointment. He did not run an ECG, although they have them at this practice and can be used on a routine visit.
Two hours later she had considerable pain at rest for the first time and my father called an ambulance. She suffered a fatal heart attack in the ambulance and could not be revived despite the medics' best efforts.
The coroner has now said that she would have had a heart attack approximately 4 hours before the fatal one. This means that she is likely to have suffered an attack  - maybe silent - prior to her visit to the GP.
The question that will not now go away is: if the GP had done an ECG at 5pm, would / could this have picked up the irregularities from a previous heart attack? If so, the next question is whether medical intervention could possibly have avoided this tragic event.
My mother was otherwise in good health apart from a few aches and pains, aged 69, neither smoked nor drank alcohol and watched what she ate. I realise that asking these questions will not bring her back, but all of my family are now wondering if more could / should have been done. We need to know, if only for closure. I will be speaking to my own GP about this but would be very grateful for any advice. Thank you.

[Cleveland Clinic]

Usually when a patient is complaining of chest pain suggestive of a heart attack or angina  the physician's evaluation usually begins with a physical exam and an EKG. It may have shown a prior or ongoing heart attack, which could have prompted him to take her to an emergency room instead of going to home. Alternatively, it might have not, and she could have developed the heart attack on her own afterwards, it is difficult to say.