When a premature beat is difficult to identify (SupraVentricular vs. Ventricular), and you have scrutinized for atrial and QRS morphological data, a useful "tie breaker" is the rule of compensitory pause.
Step 1. Measure the R to R interval of a normal sinus beat and double it. (It is acceptable to find three normal QRSs and set your calipers.)
Step 2. Then place the left edge of your caliper on the Normal Sinus QRS before the "unknown" (premature beat) and look where the normal sinus beat after the unknown comes in with respect to the right caliper edge.
If the normal sinus beat comes in before the caliper point, you have a less than full compensitory pause. This favors a supraventricular premature beat.
If the normal sinus beat falls on or comes after the caliper point, you have a full compensitory pause. This favors a ventricular premature beat.