How can I tell the difference between an angina attack and a real heart attack?

If you are using 3 Nitro to treat an episode of angina and not getting relief, you need to call the ambulance. If she is getting relief with 3 or fewer Nitro but has angina later you need to talk with her doctor as soon as possible or phone the ER or a nurse help line in your area for middle of the night advice. There are long acting medications that are taken on a scheduled basis to prevent angina that may be of benefit to her. Ask her doctor about those.

P.S. In the moment you can't tell the difference between angina and a heart attack.

It was my experience that the ER will not give advice over the phone. They just say something like, "If you feel it's an emergency you need to bring her in."

The 3 nitro rule is the rule of thumb. If she's needing more than 3 nitro's she needs to go to the ER.