Hi Guy,
I' ve had the same problem in the past. Generally i let them stumble through their introduction then, ask them if they are jws. At this point i usually say... oh i used to be a jw.. they will then say oh really why did you leave... enabling you to say for example.."well a few reasons but i never quite understood the generation and 1914 could you explain it for me".
I find this method easier than just dropping a bomb on them... doing that tends to set their apostate alarm bells ringing. Let them feel they are in control, answering questions with questions is always easier.
Elhoim.