I learned this trick this week from Phoenix of Enlightenment Dating. And it's not just for dating either. If you have friends that are regularly late or flake just tell them to text or call when they're on their way. That's when you know to actually get going. If they don't call then do something else. If they later show up and complain you're not there then remind them they were supposed to call/text. In a dating sense, if you're not sure she's going to flake or not just do the above. If you don't hear back from her then you know she's flaking. It beats going to a restaurant or something and watching the candle burn down wondering when/if they're going to show up.
Furthermore, something else I stumbled upon a while back when setting up a date/day2 is to give the person just enough info about when or what you've got in mind. The trick is to sell the event while leaving out enough information so that you have a reason to call the day of to finalize the details. This serves two purposes. A) it gives you a feel for if they'll flake based on their tonality and excitement level and B) leaving details out gives them less info to base their decisions on when weighing their options. There is less chance of them choosing something else over your plans if they don't know what your plans exactly are.