Global ST segment elevation (seemingly not confined to a vascular territory) is a differential clue favoring pericarditis over STEMI. The rule, however, is not absolute, because very proximal occlusion of a wrap around LAD variant would threaten the anterior, inferior and lateral walls, leading to widespread ST segment elevation.
Accompanying a tracing showing pericarditis at Dr. Smith's ECG blog is a discussion of other electrocardiographic features in the differential. PR segment depression is a well known clue to pericarditis. Less well known is a QTc of at least 372 ms in nearly all cases of anterior STEMI and reciprocal ST segment depression in nearly all cases of inferior STEMI, features not characteristically present in pericarditis.