A remote, small heart attack can be difficult to detect. The best test for making the diagnosis depends on the particular lab. We are fortunate at the CCF that both our nuclear lab and echo lab are outstanding; we would use either one to diagnose an old, small heart attack.
The accuracy of the tests depends on the size of the heart attack, the location of the heart attack, and when it occured. Additionally, such things as body size/shape, motion during the test, and other factors affest the accuracy. In general, the accuracy at our institution would be on the order of 80-85%.
Cardiac MRI has not been extensively studied for diagnosing heart attacks, and should not be considered the standard. Few centers could do this study well; and even then the results might be questioned. Cardiac MRI is most often used to study the overall shape of the heart, look for constrictive pericarditis, or to diagnose hibernating myocardium.
If an old MI is diagnosed on ECG, it is present for the remainder of your life in about 90% of cases.
Hope that helps.