It depends. While ST-segment depression is most certainly a common sign of ischemia, it is not always definitively caused by ischemia. So the ST-depression might very well be baseline, while not a sign of an ischemic condition.
Hypokalemia, digitalis effect, left ventricular hypertrophy, bundle branch blocks or other infranodal conduction delays, and mitral valve prolapse can all be potential factors in ST changes; any two given cardiologists can often argue about any one given ECG at times. Prinz-metal angina and other vasospastic conditions can cause an acute-on-chronic ischemic change in ECG; though Prinz-metal usually causes ST elevation.
Definitive diagnostic testing is usually required to provide a cause; it's why we do enzymes and other tests as well as ECGs. Remember too that ECGs miss a rather substantial percentage of MIs - so not so much a definitive marker of ischemia/infarct, though they are useful to raise the index of suspicion.
Certainly an interesting question and one worth researching. I recommend this website among others: The University of Utah's ECG Learning Center