For me i began this critical thinking of WT publications when i started college. I started studying a bit of philosophy, which makes you question even the most basic. It always amazes me when JW's quote a particular scripture (can't think which one) that basically says how people went out to check for themselves the teachings. They say that JW's must do this too, but only within the confines of using WT publications.
When i started university my critical thinking really kicked in and made me question everything that i was taught.
Paul