Keeping in mind that I dont compete, and generally dont attempt max limit tests, it's a daily discovery thing, not something that I plan for. I recently hit two maxes significantly above what I thought I could do, mainly attributable to my not testing such maxes regularly. In both cases my general thought was "I've been feeling pretty good the last few weeks/days, I'm going to go heavy today." Then, when I got to my planned top set... it felt like I had significant margin available... so I went for what turned out to be personal bests.
The downside of this is that I'm not sure what, specifically, in my training led to the advances. I have ideas and theories to test, but nothing concrete.
So... no help in how to plan or program for such a thing. Sorry!
The key indicators for me though are lack of pain anywhere and a feeling of strength in my lower back and grip (forearms). That is, going close to what I thought my limit was, and not having those parts register any "complaints".
I'd be very interested in what others have to say about programming for such though!