You ask a difficult question as heart attacks can manifest themselves in atypical ways. Some heart attacks can even be devoid of chest pain (i.e. a silent heart attack). Some will have chest pain, others none, and even others will manifest as abdominal pain, shoulder pain, jaw or back pain, etc. It may not even be pain in the classic sense. It can manifest itself as a 'tightness' in the chest, an 'ache' or 'pressing' sensation. The fact of the matter is you won't necessarily know if you are or aren't.
If you find yourself getting pain in the chest, especially after exertion, call 911. If the pain hasn't resolved after rest or after a Nitro pill, call 911. If the pain is associated with shortness of breath, sweating, nausea, weakness or dizziness, call 911. Shortness of breath (with or without chest pain) call 911.
In short, call 911! Especially with your preexisting history.