Yeah, you have to get down to the nitty gritty of what people are saying. Someone says they believe in Jesus. Well that means nothing until you start getting down to the fine details of what they mean by that. Same for "justification by faith" as you said with your colleague, and everything else. It can be complicated. But since the simple gospel is not complicated, usually the falseness starts to manifest itself pretty quick. That's what I've always seen at least.