You have the expressions sprite.count and sprite.pickedcount to compare, for this.
First you make a pick by comparisation condition. Expression will be Sprite.Bullet.Speed. Object = that Sprite.
Now it picks all those instances who's bullet speed is zero.
Now you want to know if they all have a zero speed. Just check if they are all picked.
A sub event Compare two values. Value 1 will be Sprite.PickedCount, = , value 2 will be Sprite.Count.