Just look for the sacs to develop and separate from each other. There’s no tell tale sign of when the sacs will open, to my knowledge. @imSICKkid and @MattyBear have done some breeding so maybe they can clue you in to the signs or at least the harvesting method.
You’ll see the balls start to droopy and they will open up releasing the pollen. Usually around 4 weeks they start opening up.
What @imSICKkid said