Can a ekg or holter monitor show past heart attacks?

Yes and No: EKG can show some changes in Q and S waves and ST. Elevation may suggest old myocardial infarction. Hotter monitor is a device that registers heart rhythms/ rate for an extended period of 24-48 hrs. It's main use is to diagnose various types of heart blocks.

Sometimes: an ECG can show evidence of prior heart attack. A Holter will only show current heart rhythm.

Yes: Most times the ekg can show signs of old or previous heart attacks . A 12 lead ekg is a good start . More definite results can be seen on ECHocardiogram

MI/ CAD: AN EKG is the first step and will show evidence of past Q wave Heart attacks. Subtle changes may suggest early primary artery disease. Then a stress test or stress thallium is done to define the anatomy. A holter/event monitor is done to evaluate palpitations or syncope .