I think a big mistake that some make in trying to get a JW to see they are not in the one and only true religion is to argue about Bible doctrine.As soon as you use the any Scriptures they see it through WT interpretation filter, only the most open-minded will be able to entertain an interprtaton contrary to WT, and with 5 meetings of indoctrination a week very few are that open.
Another approach may be perhaps better, it will not have very quick results, but it could put a crack in the thick shell the WT has formed around them.
Here the approach I used at the mall when witnessing to the JWs (two women) at their magazine stand.
As I was looking at the books I mentioned that my son-in-law was a JW (would not tell them that I used to be one). And also that they are nice people but there is one thing I don’t agree with and that is that they think they are the only ones that are going to be saved and that all the other people that believe in Christ are doomed for destruction.
Of coarse they have their clever answers that are designed to deflect such objections such as "only God can read hearts" and the like, if you press them some more, which I did, they will answer to the effect that yes they believe they do have the only true religion, which they did, and then went to explain why they felt that way.
I then proceeded to give an example, of a family who believed in Jesus Christ and had a genuine faith that moved them to help the poor, be good parents, work hard, live good moral upright lives, basically follow the Bible, and I then asked them. "Is God going to destroy them just because they didn’t join your religion?" They then proceeded to make typical JW comments "you got to know the truth blah blah blah" you know the common JW rhetoric. So I asked them a couple more questions "do you think to get salvation you have to get 100% right on some Bible quiz"? "Say for instance a person like myself believes in the trinity is God going to destroy me because I guessed wrong on this subject"? "Is having this or other wrong understandings a sin that merits everlasting destruction"? I could tell they were starting to get uncomfortable. I mentioned that we all have wrong ideas about God and the Bible JWs included, and since they didn’t have the 100% of the truth, then what’s to stop God from destroying JWs as well.
The older lady mentioned that the JWs were humble and correct wrong understanding and so that is why God won’t kill them with the rest, because they corrected their mistakes. I said "you admit that your group has had wrong understanding in the past, so you must also know that you still must have some understandings that in the future will need correction". To which they agreed, "So then why is God not going to destroy JWs for wrong understanding but use this as a reason to destroy the others who have a genuine faith in Jesus"? " Don’t you see the double standard here"? The two women were getting uncomfortable.
I think the best approach is not to try and get them to see their interpretation is wrong, but simply to reason with them on the fallacy that the WT has instilled in them that to get salvation you need to get a high score of 95% or better in a Bible quiz.