I dont usually plan dates in advance unless its a girl I bang or a girl that shows VERY HIGH interest.
Anyway the best suggestion I can give you to avoid wasting time its simply send a brief short message an hour before the date when you're still at home.
Just ask her if she doesnt mind having a brief walk in a shop close to your meeting place cause you have to buy something for a friend, it doesnt really matter what she answers as long as she does it.
No answer is a huge chance of flake, if you want you can send an other one or call...no pickup or no reply and you got your answer.
They say the medium is the message, her silence is.
If she instead answers that she doesnt have the time cause she is in a hurry after the date, you got your low interest, ask yourself if its worth to go at that point, I usually eject by saying that I dont wanna stress her.