Great question! (Says Morbledainen, who is of course listening.) Answer using only the tools and skills that Opalyn has immediate access to:
It's not really what this here other tool is for, but if you just tap it against your finger and make like you're trying to enchant your finger, it'll produce a pulse of disspated magic that'll refract through the cylinder! The disharmonies there, in principle, would be something you could learn to look at and see where the cylinder wasn't exact.
However! The cylindricality doesn't need to be that perfect, in order for the vibrator to work well enough to bring somebody to orgasm. Morbledainen doesn't teach the way of perfection, but the way of knowing exactly how much imperfection you can get away with. One point to Ravenclaw for her asking, though.