I think it's getting harder to show JWs the truth - not because we lack evidence, reasoning ability, etc., but because JWs now seem to be blindly loyal to the org & GB and less open-minded . In the recent past, I was thinking it was going get easier and easier to reason with them because of the mounting evidence that JWism is wrong.
I recently tried to reason with two JW relatives who I thought would be more open and reasonable. In the past they were level-headed and reasonable, but they've changed. It seems JWs are more brainwashed than ever. My two relatives were completely unreasonable and closed-minded.
To answer your question, I guess you would have to sort of know the person's current state of mind and feelings to know whether there is an opening. If there is an opening, you could maybe present some point as if it's disturbing you and you're just seeking the JW's opinion. Maybe that would penetrate the 'force field' around his head and get him on the road to seeing TTATT (the truth about "the truth").
On the other hand, if you don't see an opening - some kind of doubt, frustration, genuine honesty, etc. on the part of the JW - it might be best to not try to show him the truth, or to let some time pass and see whether he changes, or perhaps to just briefly mention some point maybe in question form as if you're wondering about it.