When looking at an electrocardiograph, premature ventricular contractions are easily spotted and therefore a definitive diagnosis can be made.  The QRS and T waves look very different to normal readings.  The spacing between the PVC and the preceding QRS wave is a lot shorter than usual and the time between the PVC and the proceeding QRS is a lot longer.  However, the time between the preceding and proceeding QRS waves stays the same as normal due to the compensatory pause.
Premature ventricular contraction electrocardiogram
Premature atrial contraction electrocardiogram
Haff disease electrocardiogram
Intraventricular conduction delay electrocardiogram
Adams-Stokes syndrome electrocardiogram
Catecholaminergic polymorphic ventricular tachycardia electrocardiogram
Ventricular fibrillation electrocardiogram
Pulseless ventricular tachycardia electrocardiogram