Neverknew: I got the "but we're the only ones who preach" argument which I countered with the Pentacostal growth.
The JW's do not have exclusive rights on spreading the gospel of Christ to the ends of the world. They are not the only ones preaching... they are the only ones who make that claim, and repeat it to each other at every meeting.
One of my close friends is a missionary in Papua New Guinea. Our small church supports missionaries in Brazil, Guatemala, India and other countries, including an outreach to the Jews in Israel.
The JW's are the only religion that REQUIRES everyone to go door-to-door, whether this is their talent or not. They are not allowed to use the talents given to them, but are supposed to force themselves into a mold that may not fit their personality or skills. They are not the only ones who preach.
You could also counter with a question about how fruitful their preaching work is. "How many people have been baptized into your congregation, as a result of your preaching work? (not counting born-ins or other family members). In how many years?"
You will know them by their fruits. Shunning. Child abuse scandals.