One way to do this is to divide the RR interval in half and to place calipers set to this interval on the obvious flutter waves. Then, determine if the calipers fall on a deflection that is consistent with a second flutter wave.
In this patient, the deflections most suggestive of atrial depolarization are seen in leads II and V1 and there is a deflection at the onset of the QRS complex consistent with a second such wave. This suggests that the most likely diagnosis is atrial flutter with a flutter rate of 270 and 2:1 AV block.