I'm still doing this. I'm getting a little better at making sure that at least the first few questions students do are ok for them. However, every time I get to a topic I haven't taught before I'm back to square one.
I ask other lecturers to see if they've had students find a particular topic difficult but other than that I'm guessing. I can prepare my teaching materials as thoroughly as I like but just miss by a country mile with the questions I've prepared for them. I have to adjust on the fly and see what happens.
Planning and preparation is really no substitute for experience.
Getting the students to challenge themselves is another issue. Many of them don't like difficult questions and just want lecturers to tell them the answer not try and guide them through it.