I had two runs of wide complex tachycardia. Doctor said it is SVT with aberrancy. How can he be sure it is not VT?

ECG. skilled ecg readers have signs and appearances which can suggest SVT with aberrancy rather than Ventricular tachycardia. Usually it isn't very difficult if we have other ecgs on the patient to compare it with or the run in/out of the arrhythmia. Ask you doctor when you have an important question like this.