As answered before, I think it was a high control group when it enforced a recruitment work in order to sell literature. So that was pretty early on.
I would say "it was clearly apparent that Watchtower was a high control religion" when the 1975 excuse came out. At that time, they had recovered from Doomsday predictions before, but it would be hard to hide this past from the members. So they went into full mind-control mode and denied what they did and told members what to think about it.