Although not foolproof, the float test is a fairly consistent way to tell whether or not your starter is ready for baking. Gently drop of a spoonful of starter into a glass of water. You will find that either the starter floats or settles to the bottom. This indicates whether your starter is holding enough gas bubbles to bring about rise. If it floats, it contains lots of bubbles of carbon dioxide, the result of happy yeast that is ready to bake. If it sinks to the bottom, the starter is presumably not ready, requiring further time and feedings before it is ready for baking.