The diagnosis of myocarditis is primarily clinical, based on typical precursor symptoms, corresponding clinical manifestations, and physical signs.

The precursor symptoms usually occur one to three weeks before the onset, including symptoms of a viral infection such as fever, general fatigue, and muscle soreness, or gastrointestinal symptoms like nausea and vomiting. Subsequently, symptoms such as chest tightness, palpitations, difficulty breathing, and even fainting and sudden death may occur.

The physical signs generally include arrhythmias, commonly premature atrial contractions, premature ventricular contractions, or conduction blocks. There may be an increased heart rate which does not correspond to the body temperature, and there could be the presence of second or third heart sounds or gallop rhythm. A minority of patients may show signs of heart failure.

Tests can include electrocardiograms, enzymatic studies, or echocardiograms, and magnetic resonance imaging may show symptoms of myocardial injury. To confirm the diagnosis, an endomyocardial biopsy must be performed.