Its best to build a rapport but if its just a quick encounter you can ask these questions that have not to do with doctrine but will show you that they are a controlling cult.
Ask these question, play along but dont let them change the subject.
(so you seem interested)a) Is the bible your source?b) What do I have to do become a jw? correct answer - a) the bible companied with publications released by the Watchtower Bible & Track Society (headquarters). b) follow their doctrine, become a non-baptised publisher, attend meetings and do verbal discourses at the hall.
What occurs if I dont follow 1 of these? correct answer- you can not be baptised
Once baptised, I decide to change religion, what happens? Correct answer you are shunned by family and friends in the cult, if you attend meetings you are not allow to comment or have any duties at the hall. (Dont let them give "this is the one true religion", make sure you get them to answer the question directly) IF they say you are free to leave ask them the question again and make it crystal clear that you are referring to someone quiting and not simple a person who decided not to attend meetings.
And if I join, do I have a)cut friendship with non-members, b) and quit smoking? correct answer a) you are encourage to associated with people only in the "truth", this includes family, however your progress in the JW ladder depends on what elders think of you, non believers are automatically classified as bad associations and you can not have associations at all with ex-jw's-doing so can get you kicked out and dead to them. b) yes, smoking is not allowed at all.
Is your religion considered to be controversial by anyone? correct answer - (they may not be aware) pedophilia, shunning, blood doctrine/blood fraction doctrine, false prophecies, bible contradictions, lies.
How do you feel about ex-jw's? and have you spoken to an ex-jw and asked them why they left the religion? Does your religion pose any resctritions regarding ex-members? correct answer they can not speak with ex-jw's, they are not allowed to read or listen to material made by ex-jw's
Can you summarise what your religion believe? correct answer - jw is the one true religion, "God wants us to preached to the world our truth so they are saved", satan is the current ruler, the end is very near and Jesus will bring judgment to all- the good will stay alive and have eternal life-the dead will be ressurected and have eternal life-all on planet earth, only 144,000 who drank the wine and ate the breaded during Jesus commemoration are considered the annoited and will go to heaven. (there are more points but these are most fundamental, if they can not summarise this they are hiding something, usually a controlling cult can not be upfront with their beliefs)
What are the 3 things you dislike about this religion? correct answer They are not allowed to say negative things about the religion during witnessing, usually its between very close friends.
These questions should give you a good idea of ANY religion being a cult.