What does a heart attack feel like and how can you tell that it is not just heartburn?

You can't . Classic symptoms include central chest pressure, left arm or jaw pain, nausea, lightheadedness, shortness of breath, and sweating. Any or all symptoms could represent a heart attack. If you are worried you should see a doctor.
Chest pain.... Classically it is precordial chest pain, but atypical presentations are common. The only way to tell for sure it is not a heart attack is to be evaluated.