A test regime using a mono record should work. Perfect tangency would result in identical signals from each channel, exactly in phase. A signal analyzer would allow you to measure this.
Of course, on a pivoting tonearm perfect tangency is still only possible at the two (or one) null points available from the alignment protractor. Unless you're using a linear tracking arm, "perfect" stylus-groove tangency is impossible.
Of course, on a pivoting tonearm perfect tangency is still only possible at the two (or one) null points available from the alignment protractor. Unless you're using a linear tracking arm, "perfect" stylus-groove tangency is impossible.