I don't know if this will help, but it is something I use in warm up and thru my entire practice session. Take Irons' (27 Groups of Exercises) Groups 6,7, and 8. These are rapid slurring exercises that will aid flexibility and, I believe, may immediately tell you when the airstream is insufficient. His instructions are interesting. You start these off playing as many repeats as you can in a single breath. With practice your speed increases and the numbers of repeats you can do rises. You are always to stop in time to play a clear ending pause note. From there, you could go to any flow exercises that you have, in which you try to extend the amount of the etude/exercise you are doing without a breath. Good luck.