I can't answer for airbus, and I know it's not what you look for but the answer is this: Experience.
But I can give you this advice:
If you are still new to the type, observe how the automatics is doing it. Leave it in VNAV (or whatever the Airbus equivalent is called). Don't just let the auto fly it - really OBSERVE and make mental notes of when the auto slows down etc.
Do it some times and then replicate it while hand flying.