This document discusses, among other things, an external device capable of independently distinguishing between pace pulses delivered by an implantable cardiac rhythm management device to different locations of a subject's heart. In one example, polarity of the pace pulses is detected along two different electrocardiograph (ECG) vectors defined by three external skin electrodes. In a further example, the detection of a ventricular depolarization is also used to assign location information to pace pulses. In another example, characterizing information (e.g., polarity, amplitude, pulsewidth, time difference between a pace pulse and a corresponding heart depolarization) is used to classify pace pulses into distinct classes to which location information can be assigned. An ECG display/recorder of the external device is capable of annotating pace pulses or markers using the location information.