I had my elders "shepherding" call the other week, it all came out. It's all about the manner in which you express yourself, and especially if you talk about it to other JW members.
Apostasy to the elders only really comes under their radar if someone approaches them with concern about what you've talked about. That's when it's considered "apostasy" (creating contention in the congregation).
The apostacy comes in many forms - obviously the biblical version (questioning Jesus etc), and then the societies version of questioning the GB.
The elders will try to determine whether you believe that God is directing the organisation (in other words, whether the GB are blessed and God’s organisation on earth today). That was asked to me THREE times, and I kept dancing around it (saying stuff like I’m not sure yet, haven’t made up my mind etc).
At the end of the call, they wanted reassurance that I would not talk about my “doubts” to other Witnesses. That’s the key part (and the deciding factor of whether you get disfellowshiped for it). A letter to the society would be another form of “talking about it” (or “disassociating” yourself).