Being a jw simply revolves around a belief that they were the only ones to recognize the invisible return of Jesus Christ in 1914 and how they picked as the only true Christians in 1919.
It's really quite simple to show them what Charles Taze Russell didn't teach anything remotely close to what the WTS teaches today and how obvious it is they cooked the books to arrive at the understanding. Especially how they changed the understanding of a generation to make it fit together.
I know to the vast majority it's a social club and they don't care but it is the single most easy way to show the dishonest factor.
Once that seed is planted.