I'd say that there's no delination point between a regular roll/fall and a breakfall; rather, there's a spectrum in between. A front breakfall, after all, is basically just a "condensed" forward roll in the air.
As such, I would just relax, stay in the moment, and "ride" the throw with your body. Personally, I've found that there's no time to try to make those kinds of decisions during a fast throw. As Ron says above, I probably wouldn't try to pre-"select" which fall to take but, rather, let your body move appropriately to the situation.
As far as exercises go, you might want to grab a senior person at your dojo for some (a lot) of jiyuwaza. As long as your partner can moderate his/her techniques for you to take a range of falls from regular rolls and falls to breakfalls and work with you so that your body develops the ability and comfort to feel and move appropriately, I think you'll soon develop the type of sensitibity that you'd like.
Hope that helps.