.. the most common test, providing your starter is made using predominantly white flour, is that it will float if popped in a glass of water.
This however is not the only factor. Yes it needs to float but just because it does float does not mean it is at it’s perfect ripeness. So, the second thing to do is to smell it and taste it. Please don’t pull a not likely face, this is a totally organoleptic experience and you need to be intimate with your culture. You only need about ¼ of a teaspoon. It should smell lactic, wheaten and yogurty and still a little bit floury. It should taste lightly yeasty and sweet, then you will get a slight tiny tang. It should almost be undetectable. Then, depending on the strength, and the extraction rates of your flour, you will find a tiny bitter back note.
Your leaven will float even when it has gone too far. It will float, even when it is too acidic. You can taste the acidity. It has a citric quality. Like Lemons – light and tarty. The Day method is actually quite forgiving of some acidity, and I rather like the tang it brings to the bread. The overnight method though is not forgiving. The acidity will on the one hand give you that wonderful sour flavour, but if you are fermenting your dough for long periods in the fridge then the hetrofermentative bacteria will do their work anyway and your sourdough structure can become compromised.