Helping them to follow things through to their logical conclusions helps. For instance asking "so if" questions that look at something from several directions like - So, if JWs are "the only true religion" does that mean all other religions are bad or false? In what way?
Because, the reasons JWs claim other religions to be "false" are the very same reasons JWs are also "false." Directing WT's own reasoning back on them can help highlight flaws in the reasoning and gaps in the logic in the mind of the questioner. The absolutes WT boldy spouts should make sense both ways backwards and forwards and they don't, they just don't.
Also, get them in a habit of paying attention to words and their definitions in the context because WT as well as news etc can and does spin info to fit a narrative. Asking who wrote something and why, what is the purpose of the article and who's interest is it in.